Uninstall without an MSI file
windows, windows-installer
Solution
Take a look at a tool called MSIZap released by Microsoft.
UPDATE:
MsiZap.exe has been deprecated for quite some time. Its use is unsupported on all recent versions of Windows, and it is considered a very unsafe tool to use.
I added a link to a newer Microsoft support tool designed to clean out installations, but this tool also appears deprecated at this point. I have removed the link from the comments section.
Perhaps try the tool linked to in saschabeaumont's answer below.
Problem
I often get a problem with Windows Installer trying to uninstall a package, but it complains that: The feature you are trying to use is on a network resource that is unavailable. Is there a known means of uninstalling such packages when the original MSI is simply not available?