I actually got one of these calls about 2 weeks ago. I was in a particularly good mood so I let them go on and on about how my Windows PC and my IP address was sending out viruses, and they could remotely log in and fix it, blah blah blah.
So I played along for awhile and pretended to be concerned, and asked what should I do.
They started telling me I needed to provide my credit card information, and some information about my computer, etc., so I finally had enough and said...
"I am now really, really concerned, as you are telling me something is wrong with my Windows computer, but all I own is an Apple Computer. I am on OS X Tiger or some kind of dangerous animal I am pretty sure. So something must really be wrong if it is now saying it is a Windows computer!!. So please tell me what should I do now?"
There was about 5-7 seconds of silence, I could hear papers rustling in the background, then I just heard "and a good day to you sir" and they hung up.
So yeah, play along, and then say you have a Mac. Should confuse them enough to leave you alone for good. At least it did for me.