Error Messages & what they mean

Win98 OS on- 2 pcs, one with 192.0 of ram the other with over 370.0 of ram 20 gig and 30 gig HDS. Every so often we receive error messages:
1.) Memory Stack Overload
2.) Low on memory resources.

What causes these two messages? (The PC that said low on memory sources would not print as result of that message.) And what can I do to fix?