Microsoft Office 2007 Professional won't validate the license

I have a Windows 7 laptop where Microsoft Office 2007 Professional has been installed multiple times, each time on the same laptop. The laptop is used by inexperienced users and has been set back to factory defaults a couple of times.

The last time it was re-installed, I get a message that license key is not valid. The software is usable but the user gets a validation message every time they open Word or Excel.  I think Microsoft put a limit to the number of times the same piece of software can be installed and validated. Is there a work-around so the user doesn't get a validation message every time the software is opened?

This is a non-profit organization with a lot of temporary operators.