I think I have a "fake" virus scan software/virus thing: What do I do?

[sleepydsm]

So I was browsing the web like normal and all of a sudden I get this "Antispyware XP" thing that pops up. I'm pretty sure I never had any software like that on my computer before. I believe it its malicious software. Anyone know what I should do, or how I could go about getting rid of it? haha, can I just pay someone to make it go away! *Cries*

Thanks!

-Kevin