I'd say it also depends on what you're running windows for. If you needed windows for any application of business or home finances, anything where you store or send personal sensitive information about yourself, then yes, definitely get anti-virus software.
If you're just using windows to play games, I wouldn't bother spending money. Just turn on Windows firewall, and get a free anti-virus checker like AntiVir if you really wanted to.
As of yet, Macworld magazine advises that there are 4 prototype viruses for Mac OS X; meaning that they are pretty much concept only and are not propagating in the wild. Apple has already brought forward security updates that render them powerless. Compare this to 70,000 + windows viruses. You are virtually completely safe for now, don't bother with OS X anti-virus software or spyware detectors. They'll more than likely slow down and screw up your system, like Norton, which installs all over the place.