Hi
Any chance that you have a restore point before you moved the data to Widows 7?
If you could get back to that point in the registry you could probably just delete all the files and then install the 64 bit version.
Have you tried installing the 64 bit version and directing it to a different location on your computer, I have Itunes installed on my D:\ drive. Sometimes if it is in a different location Windows will see it as a different application altogether and ignore the old one.
You could manually go through the Registry and remove every reference to Itunes, and hope you get the one that blocks installing the new version. To do this just use the search function and let it go from reference to reverence deleting them as you go.
If you do this make a restore point first.
I have just had a similar problem, after uninstalling Norton Ghost because it wouldn't let me run it after installing a new external hard drive, and reinstalling it it will not give me permission to run it, (I get a message that says "You do not have sufficient privileges to run this) after a week of trying to get it to run I have had no luck, and it's all installed! This sort of thing drives me crazy.
As a last resort you could just delete the whole Itunes installation and then run the registry cleaner in CCleaner and hope that it removes all the references that no longer apply.
Probably the only sure way to do it is to reinstall Windows 7 which would mean you would have to reinstall all your software.
http://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
Mike