Warf Rat:
Proceed with caution! If someone "gave" you the disc/key, it's likely to be a pirated version. Unless it is a microsoft disc, you may be asking for viruses/trojans to be "graciously" included with your disc. Asking for a key here is probably against The Rules here, also....
1) If you used the trial version, the key provided by your friend (assuming it's valid) may not be compatible with the trial version....uninstall the trial version and install from your disc.
2) Other than that, if you believe the disc/key given to you to be valid/legal, go ahead and call Microsoft....
Else, pony up the cash & buy yourself a copy of MS Office. You might also consider some of the open and almost-open alternatives that have been getting quite good in the past couple of years:
OpenOffice.org
NeoOffice (Mac)
StarOffice 8 (Not quite free, but for $69, it's priced right!)