Depending on how you tried to re-install XP, you may have wiped the linux partitions. Try booting into rescue mode with the linux cd. Then do 'fdisk -l' to list all the partitions. If you don't see any that say linux (or linux related like reiser), then you likely hosed your linux paritions by reinstalling XP. If you still see linux partitions, then you might just need to re-install grub. Please post the fdisk output though.
If you did hose linux, then start from scratch and do it properly, format the disk and install windows first (leave room for linux and make sure to note where the paritions are on the disk). Then install Linux last and during the Redhat installation, make sure not to erase the windows partition, make sure it recognizes the windows partition during the bootloader installation screen (should show a list of partitions, including the windows one). If it shows windows in the wrong partition, you can simply change it during this screen or you can modify grub.con later.