None of the solutions that come to mind are elegant. The first is to use a third-party program to shrink the existing partition and create a new one. I don't know how much data you have on your system, but if it is not much, then you could also clean install Win 7 RC, use its utilities to create another partition, then load XP into that new partition.
Personally, I would opt for the third-party route. You are less likely to lose any data, and there are some excellent freeware programs to accomplish the task of repartitioning.
In any case, while Vista/Win 7RC can coexist with XP, it must be set up right to make it easy for the user. EasyBCD is an excellent (and free) way to create a multiple-boot solution. Read their FAQ pages about XP.