Well, you could use something like partimage to back up your whole linux partition just in case. Anyway, to the meat of your question, this rescue disk has saved my bacon more times than I can count. You have to walk through a bunch of funky text menu's but it works. It'll go find your linux partiton, and reinstall grub for you, with all your old settings, or if you choose, redo grub altogether, or just install the windows loader for you.... it can do it all.....
http://supergrub.forjamari.linex.org/
Good luck...let us know how it worked out....
David
P.S. Dear God, WHY exactly are you getting rid of XP in favor of Vista ? Vista is a big fat nearly dead dog. It's hideous. Well....good luck....sorta....you are going to need it...
P.P.S. Oh, I wouldn't destroy any partitions if I were you, just instruct vista to use the xp partitions and overwrite the windows stuff, then you won't have to screw around with your partiton table, or updating /etc/fstab or a bunch of other stuff. It'll save you bunches of trouble, unless you are planning a complete linux reinstall anyway. Otherwise, even your partitions will be renumbered or renamed, possibly in a weird order. Anyway, you'll have a whole new set of problems to solve....
Good luck...
David
