First create a restore point because you need to mess up with the registry. Go to Start/All Programs/Accessories/System Tools/System restore. Create a restore point. You can go back to where your PC was by selecting a previous restore point if needed (if something goes wrong)
Then try thishttp://www.wikihow.com/Disable-Right-Click-in-Windows-XP
To create the new value at point 5 click Edit then select REG_DWORD type.