Just wondering... but why on earth would you want to move from a modern OS that runs faster on similar hardware (well, so long as you have 1GB of ram for 32bit, or 2GB of ram for the 64bit), to an aging 10 year old OS that is about to loose support from Microsoft?
Anywho, simply put your CD in the drive, boot from it, format the disc, and install like you would on any new PC. Dual booting is another excellent option like WR2 mentioned if you have that one program that just refused to run in compatibility mode on your PC.