You can run windows on any Intel Mac that's running OSX 10.4 "Tiger" or 10.5 "Leopard."
For Tiger, Bootcamp (the bootloader + BIOS emulator) will have to be downloaded. I'm actually not even sure it's available for download anymore, since the Tiger version was only a "beta."
For Leopard, Bootcamp is built in.
All you have to do is use the Bootcamp utility to:
1. Shrink the OSX partition to make room for the windows partition (windows can't be installed on a USB HDD, or at least XP can't).
2. Get a blank CD, insert when prompted. This will make a driver CD so that your Mac hardware will work flawlessly with windows.
3. It'll tell you to pop in your windows disk and reboot.
4. Go about the windows installation just as you would on any regular PC.
5. After you have a functional desktop, pop in the driver disk and let it do its thing.
a. You must (at least last time I checked) still have an OSX install on the HDD.
b. Windows cannot be installed on an external HDD (least not the last time I checked, since XP doesn't support USB boot).
c. All versions of windows MUST be 32-bit (EXCEPTION: Bootcamp on a MacPro, the desktop tower Macintosh.)