Doctor antivirus?

Okay, Doctor antivirus keeps popping up and telling me i have 40 threats to my pc...However, I run my other virus and spyware detection programs (MacAfee) (ad-aware) and they say I'm protected.... How do I remove this "doctor antivirus" from my pc? It keeps popping up and is very annoying....when I click on the link to let doc. antivirus do whatever it is that its trying to do, it takes me to a website, asking me to buy the program for 50 bucks! Can someone help>?