you can install windowsXP on your machine.
steps would be
1) create a required space/partition for windowsXp, using fdisk in RHEL5.
2) install windowsXP on that partition.
3) after that you'll see that your GRUB boot loader is not present, only NTLOADER is present. That is, you'll be able to boot only WindowsXP.
4) Now, you need to install grub again.
5) For that, you can start your machine with the RHEL5 Disc1 and boot your existing Linux system.
6) Once you have a successful boot into RHEL, you can install grub again.Using "grub-install".