SQL Server "Linked Server" to MS Access Database

I have a very frustrating problem. I have a situation that works fine
on some computers, but not on others. I have SQL Server Express 2005
and Access 2003 loaded on 4 computers. I have set up a "Linked
Server" in SQL Server linked to my Access database. I am running
queries on t-SQL against the Access tables through the linked server.
On all of the computers, I can do select queries and they all work
great but what I need to do is run Update, Insert, Delete queries as
well. On a couple of the computers, it works fine, locally and across
the LAN, and a VPN as far as that goes. But on other computers I can
do Select queries (also across the network) but no update queries. I
get the error message "OLE DB provider "Microsoft.Jet.OLEDB.4.0" for
linked server "securedjetls" returned message "Operation must use an
updateable query." where securedjetls is the name of the linked
server. The Access database on the computer that is not working is a
copy of the database from the computer that is working and any of the
tables that I'm trying to update have primary keys, so that's not the
problem.