B2B Server on Win 2000 dies frequently

We are running B2B Server 4.0.2 on windows 2000.
Every time we start the B2B Server it dies after a short period of time. The license keys are valid so that is not a problem. Looking at the console log the last display is always:
000B18 [B2BSERV.0003.0002] Checking for expired webMethods B2B server sessions.
Also, There appears to be a correlation between leaving B2B Administrator running and it’s unexpected termination. It appears that WM dies sometime shortly, say an hour, after the B2B Administrator session is closed.