Sounds like you purchased the software from a discount site. Guessing one of those sites that sell Vista for 49.00 and MS Office Pro for 39.99. Be aware that those sites typically give out keys from a corporate account and tend to reuse keys.
I purchased a Dell notebook from a 3rd party reseller a few years ago. I got the system for 1000.00 less than Dell's site price. They system included MS Office Pro. After about 6 months my key came up as invalid.
You might try to actually call MS and get some help. I loaded MS Office on a pc which had to be replaced 2 weeks later. I had to call MS and get them to give me a new key for the install. They were actually very helpful.
If you find the copy you purchased was in fact a pirated copy, you can get MS Office and OS at a discount (if you are a student) at
www.ultimatesteal.com (a microsoft site)
59.99 for Office Ultimate
64.95 for Windows 7 Pro Upg