Any one can easily send you a virus or worm through email or just if you are surfing a certain site. doesn't have to be an explicit site. Check your task manager. If you see something you don't know and it doesn't say like system or network service. Then cancel . Then try and see if it works.If not keep doing that until you find what is troubling you. OR
[url]http://www.eset.com/onlinescan/[/url]
Free virus scanner. And it actually works. Or Get a virus protection.

Not Mcfee it sucks!
Avast

ez-trust etc etc