Help! System devices disappear

i help the computer teacher at my kids school maintain the machines in
her classroom. she has 30 win98 se machines networked to a linux box.
all the machines are patched with ms's latest and we run virus and
spyware scans regularly.

the machines are 4-5 years old and came with on board nics and sound
cards. we have disabled some of these when they went bad and replaced
them with pci nics and sound cards.

the problem is that the drivers for nics and/or sound cards
'disappear' from device manager and we have to reinstall the drivers.
there is no pattern i can see here. i would suspect a failing cmos
battterym but i know if only maintains hardware state, so what could
it have to do with software.

i'm stuck. any suggestion appreciated.

thanks.