I've been using the Windows 7 Beta and now the latest RC for many many months now, and in my view it is a clear winner. It's sleek, runs smooth, has many productivity enhanceents to the UI and is very stable even in Beta.
I tried to like OS X but try as I might, I find it restrictive, basic and too hand-holding. I prefer the flexibility of Windows and the option of picking any PC i like from a variety of manufacturers. My ASUS laptop is of much higher spec than any Mac you can get and for a much more reasonable price.
If you know how to work a PC properly you won't get virus's, i do not even run an Anti-Virus scanner. Every couple of months I'll use an on-line utility to scan my system and i've never once had an issue.
The vast majority of virus's can be avoided on PC's by using common sense and not opening clearly fraudulent e-mails. + I use FireFox.