The easiest, cleanest, and most irreversable way to reformat Windows, is to use FDISK. This will remove any drive letter, and it's contents, completely. It has been some time since I used it and I don't have a copy handy. It sould come with Windows, somewhere.
Here are some rough instructions that should work.
[color:"red"]Use at your own risk and only if you are computer literate![/color]
1) Click Start | Run.
2) Type fdisk and click OK.
3) Remove a primary partition.
4) Select the drive with Windows on it and when it asks for the Volume Label, type it (it's shown next to the drive info).
5) Close and save.
6) Install another version of Windows from a boot-disk/CD.
7) During installation, it will ask which partition to install to. Create a new one to replace the one you removed. Select the recommended file system.
8) Happy installation!
