This is kind of knuckleheaded, but I saw on RedGiant's website that InstantHD doesn't support 64 bit Vista or xp.

The knuckle-headed question is does this mean it will crash the computer, or they just don't guarantee it will work?

sort of answered my own question:

http://www.redgiantsoftware.com/support/faq/product-specific/magic-bullet-instant-hd/107/

Apparently the answer is you can do it, but no guarantees.