Dropping network connections

I have a W2000 server running TCP/IP, Netbeui, File and print sharing. It has MS Exchange 5.5, McAfee virus scan suite, Veritas Backup Exec 8.0, MS SQL Server running on it.
My clients consist of W98, W98SE, NT 4.0, and W2000. Ocasionally, some of the users during the day will get booted off of the network shares. Even though the network shares are not available to the user, they are still able to use their email(MS Outlook 2000/Exchange 5.5) I beleive that these are only the W98 and/or W98SE clients. I cannot tell what makes this happen. Any ideas on this one? Anything to check or any questions?
I do have the latest SP’s for W2000 and Exchange Server installed.