Is there any way to remove a computer virus for free?

My computer is telling me that I have an Internet Explorer virus called lsas. blaster.Keylogger or something of that nature. Every time I go to a web page, its asking me if I want to continue unprotected. My laptop has a type of program called System Security version 4.51, but when I say that I want to remove the viruses and worms, its trying to make me pay $50.00. Also, the message is telling me that someone is trying to steal my credit card info and pwords. Help! I'm kind of computer illiterate when it comes down to this stuff...LOL. Is there anything that I can download to stop this message? I've tried removing the program, but it keeps popping up.