Available at http://support.microsoft.com/default.aspx?scid=kb;en-us;290301
is a last-hope type of utility to fix a very specific type of problem that may happen through over-tweaking or corruption. MSI keep track of what packages it has installed and this utility makes it selectively forget one or more of these packages.
You may end up in this weird situation where you can't install an MSI package because of a entry in the MSI tracking database says that the package is already installed and won't let you re-install without a removal. A removal attempt, however, is not permitted or not possible due to corruption or interaction with other software. This utility breaks the stalemate by helping you make the MSI tracking database forget it installed the package in the first place. The catch is that YOU must choose which package entry to blank out. As the MS link above mentions, this utility does not uninstall the package, delete files or registry entries. You are responsible for that cleanup.