Duplicate MAC address

Recentrly there was a LAN outage on one of the floor of our ofice. After troubleshooting the problem it was noticed that that there are three PCs talking with the same MAC address.

We bought two PCs and installed in a saperate test lab. We reinstalled fresh OS (WIN2K) and changed the Hard disk (to insure that there is no software based MAC address changes). But still MAC address of both PCs was same ! The Mercury (845) mother board of both the PCs has same MAC address ! The onboard NIC is of SiS900 Fast Ethernet.
How this can happen ? How the MAC address is assigned to the mother board ? is it soft burnt (EEPROM kind of) or is it hard coded into the ASIC chips ? At PC level, How should this problem be resolved & prevented?