You (re)installed windows after, but now you can't boot into Debian ?
You need to reinstall the ["MasterBootRecord"], for of your previous bootloader :
Recover Grub
see ["GrubRecover"]
Reinstall lilo
The procedure is probably very similar to Grub, but re-running lilo.
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"] lilo
Because you have to reinstall lilo
Fixing The 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.