I had a problem yesterday where I got a virus in my Desktop PC (running Windows XP). It disabled the Registry Editor Tool (regedit.exe) to stop the user (thats me) from going and manually editing/deleting registry entry.
This useful command helped me enable Registry Editor. Just click on Start->Run and then type the following in (all as one line):
REG add HKCU\Software\Microsoft\Windows\
CurrentVersion\Policies\System /v DisableRegistryTools /t
REG_DWORD /d 0 /f
Hope this is useful to someone too