It would seem that there is a corruption of the configuration folder if you have deleted the cache file and still get this error message. When you installed the extension were you prompted to overwrite any existing files? You should always choose yes to all for this.
If your configuration folder is corrupt then you will need to rename it to undo the corruption, here is a link to a post that covers the steps you will need to take for this:
The paths will match what is listed for Windows Vista. Please give this a try and let us know if you continue to experience this problem.