I know it's horrible but there is no single cause or cure. http://support.microsoft.com/kb/324516 would be my first stop, but did you research this or were you hoping it was just one answer?
When I try to update some of my software I receive
this message. I looked in my Services list to see if it was turned on and it doesn't even show that file in the Services list. There is no download on Microsoft Windows 7 site for the Windows Installer Service.
This first appeared when I was trying to update my Java.