I'd be curious how much Ram and how much hard drive free and settings for swap file.

What runs in sandbox ?

Did you update and run a full scan after installing Malwarebytes?
Does Avira update regularly?
What if anything did you try based on your Google search and what was the result?

I'd be curious how much Ram and how much hard drive free and settings for swap file.
-- I have 3 GB RAM, and I will gladly tell you the answer to the other question if you tell me where to find it :)

What runs in sandbox ?
-- Original purpose was to run Firefox in the Sandbox to prevent - as it was explained to me - any outside script from making changes to my hard drive, but recently I haven't been using it while trying to diagnose this other problem.

Did you update and run a full scan after installing Malwarebytes?
-- Yes

Does Avira update regularly?
-- Yes

What if anything did you try based on your Google search and what was the result?
-- I haven't found anything specific to my problem. I found posts on other forums regarding compatibility problems with Internet Explorer 8 (which I don't use), or when sending mail or opening attachments, but nothing having to do with XP crashing or accounts disappearing. Other than uninstalling and re-installing WLM, I've tried nothing other than shut down all the "new" programs"

I'm wondering why you are using Windows Live Mail on a XP machine. This could be your problem. Don't you have OE or Outlook that you could use? I use WLM on my Win 7 machine, but stayed with OE on my XP machine. Sometimes newer programs just don't work well on older computers.

you mention you recently added these programs, my question is what AV did you have on it previously as well as which firewall? Did you use the specific removal tools created for the full removal of those programs before installing the new ones? If not that may well be what is causing your problems.
You can find some of the removal tools here.
Antivirus Product Removal Tools

to help in removal you may want to look at
AppRemover

Please let us know what you had previously and how you removed it, what if anything did malwarebytes find?

I'm wondering why you are using Windows Live Mail on a XP machine. This could be your problem. Don't you have OE or Outlook that you could use?
-- I used to use OE. Like I said, I have two Hotmail accounts, one for personal, one for business, so I would keep it open constantly. I was forced into WLM after being warned for 9 months that I would be unable to use OE, and I should upgrade to WLM. OE now only can access one account at a time, which wasn't good for me. I loved it, and if I could go back, I would. WLM takes up too much memory on my machine.

Please let us know what you had previously and how you removed it, what if anything did malwarebytes find?
-- I was running Avast! previously, but I had gotten a trojan, and after it was removed by someone who knows far more about computers than I do, I switched to AntiVir and added all the others. I can't remember exactly how I removed it, but I would have either used an Uninstall tool from the Start Menu, or else I would have used Control Panel. As for Malwarebytes, given the situation with the virus, it was run about 20 times over the past few months, so I really couldn't tell you :)

I will, however, read those links to see if they are any help.

Avast and Avira are both anti virus programs and would not detect a trojan, this is why you needed Malwarebytes or a similar program.

You note the problem appears when Live has been running for 'a while'. How long is 'a while' roughly? Have you tried running the computer or more than that time length without Live running to see if it slows or continues to run as it normally did prior to this phenomenon?

How much free space on the hard drive? Click Start then My computer then right click on C: and click Properties to see how large, how much used and free space. You need a minimum of 15% free space for the computer to operate, 20% free space is better.

" I can't remember exactly how I removed it, but I would have either used an Uninstall tool from the Start Menu, or else I would have used Control Panel."

that would not be the same as using one of the designated uninstall tools you have to go get those download them to your pc and then use it to fully remove the program. Using add remove from your control panel is not sufficient when removing most security programs including Antivirus programs. When done with only add remove programs much is left behind which can cause serious conflicts when new programs are installed.
Always remember when using any antimalware program like malwarebytes or superantispyware to always update the program before running the full scans so it has the latest info for the newest malware.

Also many of the trojans today are very invasive and if who ever helped you did not use some very special scanning tools to see if all of it was removed you may still be infected which could also be causing the problems.
To make sure it is all gone I would go to LzD forum to the area I am linking to start a post there and ask for assistance to see if all of the trojan you had is fully removed. They have special scans they will help you with.
Analysis and Malware Removal

Many of these infections now carry with them rootkits which are extremely difficult to remove with out the kind of assistance you can get at a malware removal support forum.

I don't understand why you cannot have two different accounts in OE, unless it's a Hotmail thing. I have three different G-Mail accounts in OE plus one other-MyRealBox. All work perfectly in Outlook Express as they also do in WLM on my Win 7 machine.

Make sure that you do not have 2 firewalls running. These will conflict with each other. Windows XP has a firewall. If you are running a firewall in one of your other aplications, turn off the Windows firewall or disable the firewall in your 3rd party application.

What JEMDANDY suggests concerning the firewalls is a huge problem these days and unless you check for it occasionally, it sneaks up on you.

Here's another possibility:
I don't know if your particular anti-virus program uses Sun Belt antivirus technology. If you check your task manager and see something similar to SBAMsvc.exe then, you are using it.

That particular antivirus is dynamic where it checks all opened files and incoming email for virus. Unfortunately, it is also a resource hog. I'm not saying other programs don't cause the same problems. I am just particularly familiar with this one since they provide programming for a lot of the antivirus packages out there. If another company is used, the below would still help you see if that is your problem.

You probably notice the hangups when you have incoming mail with attachments, especially large ones. All the resources are being used to check the mail and even a ctrl-alt-del doesn't have an impact. It actually will but it has to wait in line for it to get it's allocation of resources to launch. Sometimes you'll see your email request time out.

Of course, not having an antivirus checker for email is like walking through a mall in your underware. However, the best way to see if this is your problem is to go into your antivirus software settings and turn off email checking. Once the mail has transferred over, turn it back on. This would be AFTER you have done what JEMDANDY has suggested and still have the problem. If indeed this resolves your problem you have 2 choices....do this every time before calling for your mail or, add more memory.

It's unfortunate that we have to "retool" our equipment to use a product that is supposed to be helping us protect and utilize what we have. However, with the sophistication of malware and Virus attacks, the products protecting us from them have become more and more resource dependent, to keep up.

You also really need to clean out your temporary internet and cache files as well as cookies. This can do the same thing you describe if they get too large.