If software comes "pre-loaded" on a machine from any pc vendor they have to give you a copy of it on disc. Pre-Installed just mean they wack it on the comeputer for you, doesnt change the fact that that you have bought the software with the pc and not been given any discs....
Its like going into PC World, buying a copy of windows, getting one of the service guys to install it and then taking the discs back with them...?!
Anyway rather then spend hours trying to fix the issue i would just simply re-install Office, ya can do this by going back into the shop where you got it and demanding the installation discs that are legally yours :)
Even if you were to attempt a repair office it would still more then likly ask you for the disc at some point. If you were supplied with a "Restore CD" its possible that office was part of a standard build that was on the machine and could possibly be on that disc.
Either way give it a go and let us know how you get on :)