you can reinstall grub, and if you had the option originally, you will be able to boot both os'es, or you may have to edit your /boot/grub/menu.lst file. it is not that hard, just read the man page for grub.
i always install the bootloader in the mbr and it has never caused me any problems.
dunno, but i would prefer grub over lilo anyday.
if you have your windows xp cd, you can boot into recovery console and type 'fixmbr' to restore the mbr if you want to boot xp only.