I keep getting a Windows antivirus 2009 update window popping up on my PC saying I have Spyware on it and I need to download the above title then it directs me to a page where I have to buy it, whats going on is it legitimate Windows product or a scam. AOL Spyware cant find anything but my computer has shut down twice with "SPYWARE MONSTER FX" on screen

Is there a free Spyware program that will sort my PC or do I have to stump up the cash? Which are recommended please?