Thought I'd append my experience of the above problem - you can find all sorts of references to it everywhere.

My problem was that a userform defined with Excel at work (containing DT pickers) gave the message in the title when opening it at home. I had a light-bulb moment and wondered whether there was a difference in the version numbers for MSCOMCT2.OCX at work and at home. Turned out the work version was newer. I then copied the MSCOMCT2.* files from work, made a backup of them at home and copied those from work to my C-drive (Windows XP - c:\windows\system32\ ).

No luck. I then rebooted the machine - still no luck.
Then, finally I unregistered the old DLL via
Code:
regsvr32 /u c:\windows\system32\MSCOMCT2.OCX
(not sure if this was necessary, but I didn't think it could hurt). Reregistered the DLL via
Code:
regsvr32 c:\windows\system32\MSCOMCT2.OCX
and what do you know - it worked.

Summa summarum -
it could be an idea to check whether the two machines have different version numbers for the MSCOMCT2.OCX files.