problem with file associations

I am done file associations many times using windows explorer or using "open with". Recently ran into a problem I can not figure out. I am trying to associate AutoCad DWG files with BentleyViewer.exe. I have done so with other computers here on site without a hitch (identical hardware / software). I have that one computer that does not want to cooperate. In Windows Explorer-Tools-FileTypes I associated DWG to BentleyViewer by browsing to it. This program refuses to stick to the extension. I can associate DWG with other programs just fine. However, when I do so and when I go back to reassign it the "delete" button is also greyed out, unlike any other extension. I have gone thru the registry and deleted everything that is associated with the DWG extension and it disappears from th file extensions window. Attemting to reassign shows the same behaviour. Any ideas?? I ma suing WindowsXP, SP2.

One thing different about this computer. At one time it had another viewer for AutoCad files, don't know if it sabotaged reassignment somewhere. This other program was removed form this computer.

KP