Do you mean MSI has stopped working? There's an app that might fix that,Dial-a-fix. It's quick and easy to run, but you should follow the directions and run the recommended diagnostics first to rule out hardware problems and file system corruption - if there are problems like that, you could make things worse. If you've created a folder named "Documents" in the root of C: drive, rename it or back it up; there's a bug in the program that will delete it.
After doing that, simply open Dial-a-fix (it doesn't recognize IE versions after IE6, so you'll probably get an error message - ignore it), checkmark "Fix Windows Installer", and click "Run". It will unregister and reregister MSI, which is often all it takes to fix it. If that doesn't work, you can download the MSI installer from Microsoft and reinstall it - which is not a huge job, either.
Dial-a-fix contains many other fixes for common Windows problems - but it doesn't work Vista or Win 7, or newer IE versions. Run the part with IE fixes and you get many error messages with IE7 +.