Stop downloading ze pr0n, you dirty boy, you!

If that's not possible, then run your standard Spyware and Adware checkers (Spybot, Ad-Aware, AVG Anti-Spyware, and there's a new one called MalwareBytes which seems to pick up on everything), and do a bit of house-cleaning (clear your cookies, history, temp files, etc). It'll also be worth finding a Pop-Up Blocker.