Very rarely, I have the need to take a machine which had a Linux distribution installed on it, and need to reinstall it with Windows. Typically, it's a work laptop for which I have temporarily borrowed, and need to return it to the state I got it in.
Recently, I've had two machines which the installer crashes whilst booting the Windows XP install CD, still on the black screen. Any Linux installer will boot fine, however. It took me a fair while to rule it down to Linux having written to the MBR of the disk, and Windows installer getting confused. I didn't guess this initially, because I thought Windows would just overwrite it. But apparently not.
So, if you've got an ex-Linux boot disk, and you want a Windows XP (Home or Professional) CD to boot with it, then boot a Linux install CD in "rescue mode" (I used the CentOS 5 DVD, and typed "linux rescue" at the first prompt). Then, once it's booted and you've got a shell type:
dd if=/dev/zero of=/dev/hda bs=512 count=1
This will make your previous Linux install fail to boot; but that's fine, I wanted the machine to boot Windows only anyway.