It sounds to me as if something has been corrupted in your system's copy of either Jet, or the MDAC. "Jet" is the database engine that manages the data in Access Databases (".mdb files"), but it is also used by Word to get data from mailmerge data sources
that you create in Excel. "MDAC" stands for "Microsoft Data Access Components" I think, and is also needed for Word to get data from Access, Excel, and some other types of data source.
For MDAC, you could start here:
http://support.microsoft.com/kb/874115
which links to an "MDAC component checker" that you can download here:
http://www.microsoft.com/downloads/details.aspx?amp;displaylang=en&familyid=8f0a8df6-4a21-4b43-bf53-14332ef092c9&displaylang=en
I would check that before looking at Jet - I was only able to find troubleshooting articles for Jet 3.5, and your version is almost certain 4.
You could also attempt a "System Restore" using Control Panel backup and restore. If you have a suitable restore point just before things started going wrong, that may put things right. In theory system restore operations are not supposed to lose your documents,
data files etc., but I would make sure everything is properly backed up before attempting a restore.
Peter Jamieson