Virtual Memory

We have a Windows NT Server 4.0 with Service Pack 4 and has 256MB memory, (2) 4.3GB SCSI Hard Drives Mirrored, A/Open 10/100 325 Network Adapter. The system keeps giving us the following message:
“System is running low on virtual memory”
I’ve adjusted the virtual memory setting and it’s set at the proper value based on 256MB memory. What’s up?