There is probably a F lock key on your keyboard, hit it and then your F8 key will work again. For antivirus you can use Avast, AVG, or Microsoft Security Essentials. If you have some pirated Microsoft software though don't use the Security Essentials you will just get spammed with pop ups. I would also download Malwarebytes and Super Anti Spyware. I download them from CNET when I put them on a customer's computer. After they are installed Update them and run. While they are running I would hit Start -> Run -> type "msconfig" go to the startup tab and uncheck the Super Anti Spyware. If you don't it will bog down your computer a ton at startup. After these two scans are done, reboot your computer and then you should be good. If you think you still have a bunch of crap on your computer though you can download CCleaner. Once again my preferred method is through http://download.cnet.com/windows/
Once it is installed you just hit run cleaner, it will clean out a bunch of temp files and useless crap on your computer. Then go to the registry tab and hit scan for issues. Then when that is done hit Fix selected issues. Then restart your computer.