If you have a Linux boot cd (knoppix or dsl will work), load it, and run grub. In the grub shell (assuming partition 2 as linux partition), type "install (hd0,1)/boot/grub/stage1 (hd0) (hd0,1)/boot/grub/stage2 (hd0,1)/boot/grub/menu.lst". This will reinstall grub to the mbr, and use your existing menu.
If you have a grub boot floppy, you can boot it, then type "configfile (hd0,1)/boot/grub/menu.lst" to boot from your grub menu.
Lastly, you should also install grub on the linux partition so that it can be added to Windows boot menu, with "install (hd0,1)/boot/grub/stage1 (hd0,1) (hd0,1)/boot/grub/stage2 (hd0,1)/boot/grub/menu.lst" in grub, then adding that partition to Windows boot menu (don't ask me how, though - I don't do Windows).