Windows 98 keeps locking out

We have a 2003SBS domain with 25 clients, win98, me and XP. One particular 98 machine has a very annoying problem which occurs betweeen 2 and 4 times during a working day. The user logs in first thing in the morning and remains logged in. At a random point while the user is working with programs and files suddenly all network access is denied. The user account in active directory becomes locked out. So in order for the user to continue working it is necessary to restart the computer and reset the account lockout.
I am very worried that at some point this is going to cause data corruption if the security goes wrong whilst a program is manipulating some data.
I would be eternally grateful for any info that can help me solve this problem.