This is not an easy task. First of all, please make a backup copy of your current Master Boot Record. Then, I would recommend you to try to use the GAG boot manager. It is free software and really easy to use.
If this doesn't works, you can allways boot from the RH CDROM in rescue mode and mount your Linux partition, from there, you could access the lilo.conf, make the necessary changes in the configuration to include your new Windows XP, and run the lilo command, which will make available your dual boot again.