Pop your Windows (*provided it's running windows) CD in, restart the computer, and it should boot to the CD. You should have options to reformat on the CD... If it doesn't boot to CD, you may have to change the boot sequence to CD first (through the BIOS).
Depending on what type of CD it is (recovery or a regular copy of Windows) you may get a few different options. I have a regular copy of Windows XP and when it boots to the CD I just choose to delete the partition, create a new partition, format the hard drive (NTFS or FAT....if it's less than 5 years old you'll want NTFS) and reinstall a clean copy of windows. IF you have a recovery CD, you might get different options...I used to have a recovery CD on an old HP machine, but I can't remember the options.