This is happening a lot - I'll look at writing a FAQ.
I don't know how this happens but the problem is a corrupt registry key. Luckily Word is great at coping with registry problems so all you do is delete the key and start Word, whereupon it is re-created with correct values. Re-installing doesn't usually work because it uses the old registry settings.
Editing the registry is not to be done lightly but if you do exactly what I say and nothing else you will be OK.
Close Word (if it is open) then click Start->Run and enter REGEDIT then click OK
Navigate down the tree on the left to the following key:
HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Data
You will see on the right two values, Settings and Toolbars. Click on the word Toolbars to select it and hit the Del key to delete it. Confirm the action. Close Regedit and restart Word.
Your menu should be back, but you may have lost some toolbar customisation. That is a small price to pay for getting Word working again.
Good luck