I have installed and uninstalled several sets of bits with .NET. One thing that I have found INFINITELY valuable is the utility called msiinv.exe. This utility queries the registry for information about installed packages. It seems to be a little more accurate than the the windows control panel. Great, where do you find this utility? Googling doesn't help much. I posted it up in the newsgroup microsoft.private.sqlserver2005.setup (don't worry, its actually a publically available newsgroup). I put it under the post about "Error re-installing SqlServer Express" to assist in problems that this person was having.
You can run it and pipe the outputs to a file. From this, you can take the product code (it looks like a guid) and run msiexec /x guid. This will uninstall the offending package.