The product key for OneCare can only be used to activate a OneCare subscription, and then only one time. Subsequent activations of OneCare require you to sign in with the same LiveID as you used for the original activation.
Microsoft Word requires the product key that matches the product from within that retail package. So, if you installed a copy of Word from a CD, the key inside that box will be required to activate. If your PC came with a trial version of Office or if your CD is a trial version, you need to purchase a retail copy of the product and install it, the installed copy may not be a standalone copy of Word, it may be a trial version of Office Student Teacher Edition, for example, in which case a standalone Word key will not work.
You should go to the Office site, http://office.microsoft.com to learn more and review your support options, or visit the Office newsgroups here:
http://www.microsoft.com/communities/newsgroups/default.mspx
-steve
