if it did not come to you as a critical update and if you do not use programs that are saying they need .Net 3.5 to run then likely you can do with out it. I do only critical updates on my windows boxes. But each person has to determine that on their own.

I do not do them, I have not had any programs that required me to update the .NET framework, if at some point you do use a program that needs it the program will let you know and give you the info for which one it needs. Many of them can cause problems. If you review the patch list chart Susan Bradley puts out you can see what she recommends. problem-patch chart and an earlier one problem-patch chart

it should now be listed with the disable button usable and you can disable it in the add on list, when it was first secretly installed the .NET framework assistant did not have an option to disable or remove it which really did not go over well at all. I personally do not have it on any of my firefox browsers. Firefox actually blocked that add on for a while, this is their info on why .NET Framework Assistant Blocked to Disarm Security Vulnerability this is a list of the various addons that firefox has on the blocklist, a good resource to check and see if you have any of them active on your firefox blocklisted add-ons

unfortunately some of the .net are not backward compatible so it may be that you did not have the specific one that application wanted you to have. I try to steer clear of all the .net stuff and just keep the original 1.1 that comes with windows xp. Thus far I have not used anything that required any other .net framework.