Since the thumbnail viewer was updated most recently in order to be Vista compliant, we've noticed that some users are getting this error message when attempting to install. Check the following:
1/ Make sure you are logged on as the administrator
2/ Disable any antivirus software temporarily then attempt to reinstall the viewer
3/ Search for the file arthumb.dll and check that it's in your windows/system32 directory.
From a command prompt (or the 'run' start menu option) type:
If none of the above options work, unfortunately it appears that the current thumbnail viewer dll won't register on your system. I have a bug noted down here with the error message, and while we've not been able to reproduce it locally, we do have some ideas as to the cause and will be looking to make some changes for a future update. Sorry for the inconvenience!