I found the following after a Google search. Using the indicated DWORD registry key, you can enable and disable right-click context menus in Windows XP. Maybe this is the key that was erased/changed:
Step 1: Click on "Start" and select "Run." Type "regedit" into the textbox and hit "OK." This command will open the "Registry Editor" window.
Step 2: Select "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer."
Step 3: Open the "Edit" menu and choose "New" then "DWORD Value." A new file will appear on the right side of your screen with the file name highlighted. Type in "NoViewContextMenu" for the new file name.
Step 4: Double click to open and type in "1" into the Value data box. Hit the "OK" button.
Step 5: Close the Registry Editor by selecting "Exit" from the "File" menu. Restart your computer for the change to go into effect and you have now disabled your right-click feature.
Tips & Warnings
* The change will affect only the current user. To disable for all users instead of selecting "HKEY_CURRENT_USER," choose "HKEY_LOCAL_MACHINE" and continue with the above path.
* If you wish to re-enable the right-click feature, return to the "NoViewContextMenu" and change the Value data box to "0." Restart your computer.