Error: "Windows cannot start more than one copy..."

[Ken Becker]

Anyone know how to prevent the error "Windows cannot start more than one
copy of this program"?  It comes up occasionally on our Win95 and Win 3.11
Internet terminals when trying to start Netscape.  Netscape is not open
but Windows apparently thinks it is.  

Rebooting sets it straight (perhaps some sort of memory problem?),
but what might cause this?

Thanks,
Ken Becker
Menlo Park PL