Originally Posted by cuschu
If you boot from the Windows XP disk and enter the setup, it should give you the option to choose which partition to install on. Choose the one that your current Windows installation is one and reformat it.
...but after you reinstall windows, you will probably need to reinstall the bootloader (grub or lilo). So, have with you a live-cd or a grub/lilo boot floppy disk so that you can recover the bootloader. Some distributions also have a repair console, accessible from the installation cd/dvd, which can be used to recover the bootloader.
Personally, I have done this 2 times. Windows installed fine on the partition that I specified on the setup. After the installation, the computer booted directly to windows. So, I used a mepis live-cd to recover grub from the grub command-line.
Warning: When windows prompts you to select a partition, do not choose the partition according to the drive letters. Drive letters may be different during setup! (In fact C: is always mapped to the current active partition on the first hard disk. If you have only one windows installation, drive letters will probably be the same during setup.)
Check partition sizes instead to be sure.