If you are not worried about backing anything up or saving anything it is a piece of cake.
1 - put the Window CD in the drive.
2 - restart the computer
3 - As soon as it comes back up go into the bios (should be F1, delete or something like that)
4 - Set the CD Drive as the first startup device
5 - exit bios
6 - when prompted press any key to start from CD
7 - follow the prompts in the Windows Setup.
8 - make sure you delete the partition and format it to a new partition (that is the step that wipes it).
More info from Microsoft:
http://support.microsoft.com/kb/315341