I would proceed in 2 steps:
1. Get (form a friend, a local LUG, etc) a linux-on-cd (e.g. www.morphix.org
) or if you can't, at least a floppy; boot with it and backup really important data from both systems (dont bother for avi/mp3, the chance of losing anything is low).
2. Use a self-contained grub diskette to try experimenting with its command prompt. For example, to boot in winxp you should type
root (whatever the above command gave)
PS: if you successfully boot in mdk, run /sbin/lilo, this MAY fix all