There are a few software installation monitoring applications.
Though honestly - the best choice here would be something like SMS - as you can have in monitor an install, then create a package to roll it out (like you would if you needed to push a critical update out company wide for example).The key here is you just modify the package to not push files - but just the reg entries.
One big issue is that SMS isn't cheap or free. And it can be a pain to set up. If this was a already established environment it would be easy.
Thats the more "high tech freak answer" - but its also probably way overboard for what you want to do.
Try this -
http://www.epsilonsquared.com/ - its free and supposed to do what you want - though you may have to do some legwork. I have never used it - but asked a quick Q and got steered to it. Good luck.
*** Why run xp64 - ugh! Hated that thing. 32 bit was great - 64 bit XP is a quirky bugger!***