Power saver settings maybe?

>I've a site where there are 5 workstations (Win98) connected to a Win2k server. The app and database reside all on the server. Network is via a 3com hub on TCPIP.
>All 5 workstations log in concurrently. However, 2 of the PCs, after the terminals are not used for some time (say 15 minutes)(the application is still opened), when the user comes back and tries to click on any menu item or form that they last left, the system freezes. After some 2-3 minutes, Win98 displays the blue screen and all other PCs using the app will hang in sympathy. I've turned off all power saving mode / anti virus on all the PCs including the sleep mode on the harddisks and network cards to no avail.
>Any clues to what's the problem?
>Yau