Windows XP Pro
Member of Windows 2003 AD domain

One particular app (MAS90 accounting) runs EXEs that are stored on a shared
folder on the server and are accessed by the workstation via a mapped drive.
For some unknown reason, one workstation, two days ago, began to display this
message when opening the EXE:

The publisher could not be verified. Are you sure you want to run this
software?

As I recall, the classic answer to this problem has something to do with
trusting the LAN in IE. However, I do not think anything has changed on the
network.

What causes this? I have seen it happen before - a workstation or server
suddenly decides it no longer trusts EXE run across the LAN.