What would cause file corruption when accessing network

I have a Windows 2000/Server, 2 Windows 2K desktop's and 2
XP desktops.

The desktops' all login to the domain and access files
from the file server.

The 2 XP machines are suddenly having problems. They can
access the server drive but there is up to a 1 minute
delay before the mapped drive initializes and apparently
there are files that are corrupting when accessing/saving
to the server.

This is only happening to the XP machines and not the 2000.

Any thoughts ?

Thanks