Avail mem limited to 256MB msg at boot

We have a server running Windows Server 2000 with 2GB of RAM. When the system is booting I get a message saying that the Available memory is limited to 256MB. Press F1 to continue or F2 for Setup. F2 launches BIOS setup which shows the 2GB of RAM. I save and exit and I get the same message. F1 does let the boot process finish. Why is this happening and how do I correct the problem?