I know this was from an old thread (nearly two years, actually), but I wanted to make sure it was still more readily accessible. I searched for nine hours today and kept running against the same set of "fixes" which didn't fix anything.
I have Vista Home (64bit) and somehow (I truly have *no* idea), Windows Installer disappeared. I don't mean turned off or otherwise inaccessible. It wasn't there. I went back and forth to services, etc. searched (online and on the computer) and got NOWHERE - until I found this thread: Repair Windows Installer Service in Vista (All Versions)
It was simple but I never even thought about it. I followed the steps with one exception: because my msiserver service file was completely absent from the damaged registry running "sfc/scannow" didn't do anything and I had to open the services file and import the changes from there rather than double clicking the msiserver.reg file. I am very, very grateful to have had this information and I hope by posting my experience from today here, it will help someone else who's ready to drop kick their computer...