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You (re)installed Windows, but now you can't boot into Debian?

You need to reinstall the MasterBootRecord for your previous bootloader :

Recover Grub bootloader

Reinstall lilo bootloader

alternatively, you can also get a LiveCD linux distro, open a terminal and type:

 mount  -o  suid,dev,rw   /mnt/hdXX       #(XX =   linux partition)
 chroot  /mnt/hdXX

Because you have to reinstall lilo

Reinstall Windows XP Boot Record

If you actually want to to the other way around (i.e. remove Debian boot loader)... Make sure you know how to boot into Debian Later !

The page MasterBootRecord, section How to repair a broken MBR has links to Microsoft support on how to restore Windows (2000/XP/2003/Vista) boot loaders.