BleepingComputer.com: XP as a Server, suddenly workstations loading programs very slowly

Hi there,
Usually I find exactly what I am looking for, so no need to post, but not today.

I am on a jobsite consisting of the following:
1 x XPPro "application server", with an "M" drive (virtual) housing Payroll Software and Accounting Software. (Paymate and Munisoft, respectively)
4 x XPPro workstations
All systems have SP2, save one workstation which has SP3.

The client tells me that about two weeks ago the programs mentioned above started to take many minutes (up to five) to load on the client machines.

Here at the jobsite, I have scanned for viruses using Spyhunter, then reactivated Norton, finding no threats.

Have attempted to start the programs with Norton disabled, same result.

The programs load just fine and speedy quick on the server itself.

What could be causing the lag? I'm stumped!
Thanks,