Hello. I'm a support manager for these certain types of problems. Read on, for an in-depth view as to which Anti-Virus software is "best."
You'll find that picking an Anti-Virus Software goes between speed and strength. Now which one do you choose, and is there anyway to get both?
Two leading and well known Anti-Virus Software programs are Norton Anti-Virus and Spyware Doctor. Both of these are great programs but there are cons to both.
Norton in most ALL cases, slows down your computer dramatically to the point where the only thing left to do is to uninstall Norton, which in most cases is what happens, haha.
Next, you move on to something that is supposedly strong AND fast, now you install it, and find that it is very fast, but barely scans anything, and it doesn't protect you from the big bad internet, now you uninstall it, and stroll along till you find..
AVG Anti-Virus Software (http://www.avg.com) and you know what's great? It protects you from internet, files, self made computer problems and searches literally EVERY part of your computer, and carefully deletes what's slowing down or harming your computer so you don't lose anything important, and guess what...again? It takes up barely any space on your computer, which means it's an extremely fast and nearly invisible software.
There you go, hope this helped and best of luck!