Also, if your XP partition is NTFS, and you're not interested in totally formatting everything, I would try http://sourceforge.net/projects/linux-ntfs/.
They have a program called ntfsresize that worked for me very well, and all my data was still working (even though it might be good to backup JUST incase). That'll free up plenty of room for redhat. Redhat's installation pretty much holds your hands through the whole thing. Just read up on how to partition your hard drive. Also, don't forget to make a fat32 partition if you want to share files between redhat and windows.