C:/D: drive change screws up programs and settings

I installed Vista on a separate HD (it was actually S: in XP) and of course in Vista that drive is C:, taking the place of my XP drive, as is to be expected.
But I had tons of programs (iTunes, Opera, Trillian, Office...) installed on the same drive and partition as XP, and now they all don't work so well. A few (like Opera) will start up but will have first-install settings, and the rest (like Office) won't even load. How can I remedy this? Is there a way to make those programs think they are still on the C: drive? There isn't actually room for them, so I can't just merge drives... Any ideas? I'd hate to have to reinstall dozens of programs, and then have to reconfigure them all, but it's looking grim.


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