A slight caveat: Whichever OS you boot, it will see its own partition or drive as C: unless you do some nasty and risky things which I strongly recommend against, having done them myself in XP and decided that it is much too risky.
So, if you have exactly two partitions or disks, when you boot XP then the XP system disk will be C: and the Win7 disk D:, and when you boot Win7 it will be the other way around.
Community: Am I wrong about this if Win7 was installed on top of XP?