How do you stop the pop up advising "your computor is infected"

Dangerous malware infection was detected etc, it directs me to an internet
site to purchase Spy ware

any help would be appreciated

download and run microsoft's anti spyware beta. and the microsoft malicious
software removal tool. I also reccomend downloading spyware blaster and
spyware guard. When u get a pop up, never click on it, unless u trust it.

have u been using any file sharing programs like kazaa?