Edited slightly. Well anyway I think we're all (er, softus and I) agreed that this is a bad idea and not worth the hassle. If you are finding that C is clogging up and D is full of space then have a bit of a reorg as I decribed in my previous post.
WinXP tip: Open start menu, right click on My Documents>properties and move the target folder location to D. This will mean that all files that go into My Documents end up on the D drive and C remains relatively uncluttered and slightly less fragmented. If D fills up then make an overflow folder on C.