With all the Virus's out there, there are tons of Antivirus Programs. Here are a few free ones:
Microsoft Security Essentials
There are others, but these are what I use. If something gets past, I use MalwareBytesAntiMalware to remove it. If its a Rootkit Combofix can do the trick...Fair Warning, Combofix should be a last resort prior to reloading a PC. Some rootkits hide in the TCI/IP stacks and when combofix removes the infection, the TCP/IP stack messed up.
What's that mean? Bottom line it means you're PC will no longer be able to get on the internet.