Uninstall without an MSI file
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.
Please note that Microsoft has now released an official tool to resolve these issues, without the problems that previously existed with MSIZAP.
- Microsoft Fixit: Fix problems with programs that can't be installed or uninstalled