Depending on the version (how old it is really), it should pop up a window that tells you to register and will try to do so online. Or, you can go online at MS and see if you can re-activate there. Otherwise, you'll have to call. I had the same problem with a client a couple of years ago and I ended up calling in for them.
Here's the entry page for their Office Suites:
http://office.microsoft.com/en-us/suites/default.aspx