I always wonder what it is that people do that they get all these virusses. I don't remember getting one in at least 5 years. And even then the virusscanner did it's job and picked it out before it could do harm.
First I'd have a look at what activities you do on your computer.
Do you play games a lot or run a lot of native windows programs for which you can't find any substitutes (if you've even looked. There is a VERY large open source community with LOADS of programs out there) then stick to windows....back up what you can, scan it very well and re-install the complete shebang.
Do you use wordprocessors, e-mail, internet, maybe some light photowork and/or audiowork.
Get a version of Linux, like Ubuntu, Linux Mint of one of the many other flavours. They're great, not at all difficult to come to grips with (the basics at least), work faster than most windows versions (at least on my PC no windows has ever run as fast as linux the time that I had it), are more secure and are completely free.
For your average PC user that doesn't do any hardcore work I highly recomment Linux over windows.