I have used Windows Defender since the beta version came out, and haven't had a problem since. The only thing is, it blocks most auto installs and auto downloads with non-standard file formats...like pepakura and flash.
Don't download a free virus/adware program unless it is recomended by someone you know that is knowledgeable of computers, because most of the free ones have adware in them. Go buy one, or try a trial of one that has auto updates.
Virus scanners that do not auto update are almost as bad as not having one at all.
Another option is to take it to a computer repair place. Most of them have a base fee of $15-$25 just to look at it. Virus/Adware removal is usually around that price as well.
P.s. Make sure your firewall is always on.