Gonna make this a lil lengthy, but that's because I'm a computer nut and I try to avoid viruses as much as possible without sacrificing performance...does this mean I'm right and everyone who disagrees with me is wrong? Nope. Just my opinion. Take it for what it's worth.
AVG - ONCE UPON A TIME...it was great. The best...even better than many programs that you had to pay for. But that time is over...now it's not bad, but it eats resources for breakfast, lunch, and dinner. If you want to your computer to scan while you're using it, forget it...unless you go out and buy some super high end machine. AVG is a resource hog and while it does a good job at what it's designed to do, it brings your computer down to its knees in the process.
Avira - Great program for the most part...but I have had newer viruses slip past undetected until AFTER the computer had become infected. I knew the file was suspicious...I manually ran a scan on it. Clean. Opened the file. BLAM! Virus warnings... computer was infected out the wazoo. Ended up just formatting it. Also, it uses a lot of resources while scanning (not quite as bad as AVG though). Lastly...Avira is ad-supported and you have to deal with a pop-up ad every time it updates. It's only once daily...but it can be a very slight annoyance. Oh and their update servers can be very unreliable with the free version...especially after the release of a major update.
Avast - My personal favorite.. I have had the least amount of viruses slip past while using Avast. It can be tweaked so its presence is barely noticeable at all. I have it update and scan my computer daily, but I honestly cannot tell a difference in performance when it's scanning and when it's not scanning. I'm sure there IS a difference, but it's small enough that I can't tell during normal computer use.
For spyware/adware removal there are only 2 programs I utilize...
Superantispyware - personal favorite and typically the most effective
MalwareBytes Anti-Malware - not usually as effective as Superantispyware, but often catches what SAS doesn't catch.
Hijack This! is nice for people who know what they're doing as well...but isn't for the average Joe.
This is a GREAT site for getting reviews on various types of free software...check it out...
From that site:
Reviews on Best Antivirus: http://www.techsupportalert.com/best...s-software.htm
Reviews on Best Spyware/Adware Removal: http://www.techsupportalert.com/best...re-remover.htm