Well, I go into the registry as infrequently as possible and never recommend it to people and never had the problem you describe.
However, I have heard of these steps having been taken that, if I am recounting them correctly (big "if" here given that it's the registry we are talking about), have worked for some people, but not all.
The first thing I would do is make sure I have a backup program on my computer like Roxio (now with Norton I think).
Then I'd reboot the computer.
Then go into Windows Explorer, Windows folder, Temp folder, and delete all files there and all files in any subfolders (there might be none).
Close Explorer and from the Start button hit Run.
Enter
"Excel.Exe /UnRegServer" (without the quotes) and hit OK.
Enter
"Excel.Exe /RegServer" (without the quotes) and hit OK.
Open Excel and create a new workbook. Go into the VBE and Insert a new userform.
In the VBE, take a look at Tools > References and there should be a reference selected for Microsoft Forms 2.0 Object Library.
If that is the case, then open your workbook that's been giving you the problems and see if it is OK now.
If it is not OK now, sorry, I'm out of ideas on this one.
Todd - - be very cautious with this. Maybe copy your file to an alternate junky computer to do the experiment from there. Any problems, close down the computer, and when you restart it, engage the GoBack software to return you to 10 minutes ago so you won't be any worse off than you were if you follow these instructions, which (unabashedly in disqualifiying double-talk) I am posting but not necessarily recommending.
Good luck, be interested to know how it turns out.