One extra thing.
If you have done a freash install of Windows, and you still have all your programs on the external hard drive, there is NOTHING you can do (like update the registry) to get those programs working.
These programs all need to be installed again from scratch (onto the C drive) so they can be registerted with this new install of Windows.
And as a suggestion, it may be best to ask about something BEFORE you do it, than to just do it, and then find you need a Windows reinstall.