I've solved it like this...

If you've got access to a Windows XP machine, copy the files for Remote Desktop 5.1.2600.2180 - mstsc.exe and mstscax.dll (and the help file if you want) onto your (Vista) machine (I've put them in a different directory). Then run this mstsc.exe and you should find your problem is solved. If you've upgraded the XP machine to version 6 of remote desktop (like me) the 5.1 files should still be on there just search for them under C:\Windows.

Hope this helps,

Andy