as far as I know of, there are a few options
if you're not booted into XP "DIRECTLY" ... meaning it actually entered grub menu.
try keep pressing button "e" or "c"
right after the bios menu (you can just keep pressing it when bios started, since bios don't usually use those two keys)
and it should either let you edit grub entry, or enters into grub command interface.
From that, look online for grub commands and you can boot into Fedora, and you can edit menu.lst from fedora.
use a LiveCD, or installation DVD and enter Rescue Mode , or some shell mode
and edit your menu.lst under "/boot", and you might, or might not want to reinstall grub.
boot into XP, install ... uh ... ext2 IFS into your XP, and then mount your "/boot" on XP
from that point, you can edit your grub's menu.lst
if your computer is actually booting into XP DIRECTLY
your easiest option is to follow option b)
LOL... I totally forgot about telling you to edit the "timeout="
for that someone has already gave an answer