Microsoft Knowledge Base Article 290301 contains the following summary:
Microsoft has updated the Windows Installer CleanUp Utility. With the Windows Installer CleanUp Utility, you can remove a program's Windows Installer configuration information. You may want to remove the Windows Installer configuration information for your program if you experience installation (Setup) problems. For example, you may have to remove a program's Windows Installer configuration information if you have installation problems when you try to add (or remove) a component of your program that was not included when you first installed your program.
The Windows Installer CleanUp Utility does not:
• Remove Windows Installer.
• Remove files of any programs that are installed by Windows Installer, such as Microsoft Office 2003.
The Windows Installer CleanUp Utility does:
• Provide a dialog box where you can select one or more programs that were installed by Windows Installer. You select the programs on the Installed Products list in the Windows Installer CleanUp dialog box. After you make this selection, the utility removes only the Windows Installer configuration information that is related to those programs.
• Remove the files and registry settings that make up the Windows Installer configuration information for programs that you select.
If you use this utility to remove the Windows Installer configuration information for your program and you plan to reinstall the program, you should reinstall the program in the same folder where you originally installed it. This prevents duplication of files on your hard disk or disks.