|I'm not sure what experience you have regarding Windows installations, but here's a brief rundown:|
1) Back up all your info (CD, USB, etc)
2) If you don't already have one, get a copy of Windows 2000 on eBay windows 2000 | eBay
3) Insert the disc into your CD drive and reboot the computer. If your computer is new enough (1998 and later) it should be able to boot right off the CD.
If this sounds complicated you should use this step-by-step tutorial. The link I've included is an archive version; that way in case the linked page is ever taken offline, future users accessing this thread will be able to view the pictures and follow the instructions.
iMac G5 1.8GHz "Hero" PowerPC
2 x 1GB DDR PC3200 SDRAM
160GB WDC WD1600JD-40GBB2
Mac OS X Leopard Version 10.5.8