problems with Windows 2000 professional

I have three machines, two desktops and one laptop. All are running 2000 Professional.
All 3 have the same ‘trouble’. I’m running Norton SystemWorks 2001 on all three also. I added this hoping to ‘fix’ the following problem. No fix, 🙁
Several times a week on each machine the systems freezes without an error message. Most often when you go to move the mouse but before you can ‘click’ on any item. You can’t ctrl-alt-del to task manager. You must hit reset! I have applied the windows critical updates. but still have the problem.
All three are using TCP/IP over LAN and modem. Two have Intel pentium processors, one has Athlon.