If you are worried about your computer infected with a virus, I would highly recommend that you install the following open source programs:
Microsoft Security Essentials- download it from microsoft website. It will get rid of viruses, spywares, etc...
avast- free antivirus
Remember you should only have 1 antivirus running in your computer.
Secunia- search for it on google.com This program will search for vulnerabilities inside your computer.