Install and dual-boot 32/64-bit, but retain drive letters

My PC is set up to dual-boot XP and Vista. XP is on the C drive, and I want
Vista on the D drive. (This is how I set things up when I was doing beta
testing.)

My understanding is that if I want to install Vista to the D drive, I MUST
run the install from within my existing Windows install, which is XP.

However, my XP install is 32-bit, and I want to install the 64-bit version
of Vista. As a result, I can't run the 64-bit setup executable from within my
32-bit XP OS, so the ONLY way I can see running the install is from within XP.

So, how can I install Vista to a partition that I want labeled D: instead of
C:, if I can't launch the setup from within my existing OS? Alternatively, is
there a way to launch the 64-bit setup from within the 32-bit XP install?

Thanks.