Instead of a clean re-install of 7, you can use EasyBcd, a very nice piece of freeware, to make your Win7 boot sequence aware of the presence of the XP installation. This way, when you boot off the Win7 drive, you will be presented with a choice.
And there are quite a few boot managers that deal with the issue before you get to any bytes installed by Microsoft, but I don't have experience to help you with them.