I'm not sure how to disable the right mouse click just for the start button, but you can disable the right click all together. You have to be running service pack 2 or higher. I hope this helps.

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Start the registry editor (regedit.exe)

Move to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer

From the Edit menu, select New - DWord Value

Enter the name NoViewContextMenu and press enter

Double click the new value and set the value to 1 and click OK

Close the registry editor

Logout and Login again


To remove this, just delete the value NoViewContextMenu and logout and login again (or set it to 0)