The Windows Installer CleanUp Utility can help you remove installation files from programs that are currupted or did not completly install.
If you get a message that says something to the effect of “this program is already installed”, try this utility.
The utility says Microsoft office but it can be used for other programs, although it is not recommended that it be used with Office 2k7.
Even if the utility does not work, it’s worth a shot before hacking up your registry.
I was able to use this to fix a problem with Verizon’s VZAccess manager and the Sprint connection program for Blackberry.
More info on the utility can be found here.
The utility can be downloaded here.