try the Trial and then use your legitimate key.
Try Microsoft Office 2007 - Download Microsoft Office Software
I assume this works since MS will send a key if you want to purchase it.
If it doesn't work just ring the activation center (activate by phone) and explain the problem -- say you've downloaded the trial but you've got a valid key.
They'll give you a new product code.
In spite of a lot of negative postings that you see about MS -- they are actually quite reasonable if you explain the problem.