Hope I can help here. I've experienced this problem myself and after much googling, found that there are some registry keys that you you don't have rights to. Unfortunately, I cannot find that post again.
In this post
http://www.eggheadcafe.com/forumarch...st23840065.asp
it looks like HKCR\.ptm might be it. You can run RegMon by Sysinternals as also mentioned in the post. It might help you track it down.
Good luck.