Hmmm... I know that I've used TweakUI at one time to do just that, but the program itself seems to have gone through some changes since then. (I'm talking about the TweakUI that was coded for win 9x). It guess it's possible that you can't do that anymore now.

I don't really mind, I now just manually change the installation path from c:\program files\x to d:\program files\x each time I install something. Never had any problems doing that.