I just posted an update to a previous issue I found with this. Check my thread under Classes - VCX for details.

Check your versions of "rsaenh.dll" (XP machines) and
"rsabase.dll" on all others. Make sure that they are the latest available.

In my case XP needed version 5.1.2600.1029 of 'rsaenh.dll' to work properly. (avail in SP1)
Windows 2000 machines needed at least 5.0.1877.7 of 'rsabase.dll'.

Hope this helps.
Greg