Is your network NT Domain with workstations as a combination of win2k prof, win xp prof, etc.?
Then u have the same problem as we had in our organization way back in 2002-03, most probably, if i remember correctly the year.
Microsoft had stopped free updates and patches for Windows NT as per the life cycle support policy of Micorosft. Still patches were available for other new OSes like Win2k & WinXP. The problem occured due to problems in the Server Message Block (SMB) an application-level network protocol. Microsoft patched the same for all OSes but not for WinNT. I also used to restarted the server every morning other wise some users will not be able to login to the domain at all, they could login with only the catched profile only and not get network resources. The error for SMB was registered in the event viewer on the server as if there were to many domain users then supported, if i remember properly. You can chwck the same om your server and confirm. We immediately converted our domain to win2k3 with AD (upgrade) as our company didnot go for the paid patch support of microsoft at that time. you can also go for the upgrading to Win2k3 or go for the paid support from microsoft which i believe they would be still doing.