I think the Trend Micro (Avanquest) Antivirus (Fix-it) is one of the better products out there. Any of them will slow things down as the antivirus and malware data files grow. You could turn off some of the automated services and do them manually but IMHO, it kind of defeats the purpose. Although there are some who will disagree, I strongly advise staying away from Norton. I personally think it's a space and resource hog. I have also used SPYBOT and find it a very dynamic AV/AM program. What's even better, it is free. As with the others, if you want speed, you'll have to turn off some of the automatic detectors. Another word of caution....if you use Outlook, you will find some of the email antivirus programs will cause Outlook to fail when opening and you have to turn off email protection in order to allow Outlook to open. Trend Micro has resolved that problem with their product and I just tested it today and it works!!.

That sounds like Virut virus lets hope not though..... Unfortunately, the virus infects every .exe and .scr (actually an executable) file on your system, and when you scan with a virus scanner, it will normally delete it as uncleanable. Eventually you end up loosing more and more files. The only viable alternative is to format the system. You will need to backup your important files before a format and reinstall, but you can not backup any .exe or .scr files, because they are infected. Good luck...

AVG is good but hogged resources big time on my machine, slowed it to a crawl. Avira (free for Windows) is very good, much less a resource hog. For a free firewall I love Comodo (not the suite, just the firewall). Does a bang up job for a freebie. Even warns me of suspicious sites and if it's for sure not good (such as a google search result I click on) will warn me to stop and not enter the site. I use Malwarbytes for scanning for spyware, malware, etc. and SpywareBlaster for stopping the junk from ever landing on my PC via browsers. If SpywareBlaster is kept regularly updated, very little lands on my machine other than the odd tracking cookie.

Yes it certainly does not hurt and they don't use much battery power and have some added benefits some will help you find your phone if lost or stolen. The top 2 I recommend are Lookout security and AVAST both have free versions. They can both be found in the app stores. I would read the attributes of both and decide which you prefer. I have used both, I currently have AVAST on my tablet but have lookout on other devices.