I have never had ANY trouble with a dual boot system of WinXP and RedHat.

i have tried this many times with the variation of FAT and NTFS as the windows partition, and there has never been any 'cross over' or conflicts.

however, IF you do use 2 seperate hard drives, there is a possibility that linux will automatically use the windows hard drive as swap space, makin it unbootable.... but if you unplug that hdd for the install of linux then it wont pick up the other OS and you wont be able to create a dual boot (lilo or grub) automatically.

all in all i prefer the single hdd dual boot, however, after all of what has been said.. there is no need to change your current windoze partition. so dont.