When clicking on the vsto install link
1. Log on to the pc as an administrator
2. Locate the file UsrClass.dat in C:\Documents and Settings\user\Local Settings\Application Data\Microsoft\Windows
3. Rename the file UsrClass.dat to UsrClassOld.dat (effectively deleting but with a backup)
4. Log on with your usual username.
5. Try the MerlinXL installation again