Beware of a fake anti-virus program that calls itself "Security Essentials 2010." It will pop up on your screen one day and tell you that your computer is infected. After that, the only thing you seem to be able to do is buy the program for $29.99 (or maybe it was 39.99). It will also block you from installing any legitimate anti-virus programs that would get rid of it. It is a very nasty malware. I used HijackThis and MalwareBytes to purge it, according to the instructions here: http://www.bleepingcomputer.com/virus-removal/remove-security-essentials-2010 .

Note: This is not to be confused with the legitimate "Security Essentials" product by Microsoft, which is free, and according to my research, people seem to like it. Obviously this fake imitator is taking advantage of Microsoft's brand recognition to gain access to your computer.