Forget drive letters, they change when booting from one partition to the other. Which ever one boots, is "C". The others, you can define.
Lets call them Partition #1 and Partition #2. Both must be Primary, not Logical Partitions. Install BootMagic on #1. You can then hide #1 from #2 (when you boot to #2, you can't see #1), but not #2 from #1. Change the volume labels, so you'll know which one you're booted into, call #1 WINXP#1, call #2 WINXP#2.