I tool that has become very handy for me on Vista machines is the Windows Installer CleanUp Utility. This little program installs (looks like something to do with Office), and then can be found under the program menu called Windows Install Clean Up.

This little Microsoft program then shows you, in a little more detail, exactly what it thinks is installed on your computer.

This became handy when a machine I was fixing kept trying to install Kodak EasyShare every time it booted up, even though the normal Windows Add/Remove programs didn't have it listed. I ran the Clean Up program, and found a few extra, smaller programs associated with it installed, I removed all these and it fixed my problem.

This utility probably will come in handy for failed installations,where some of it is still in the system.