Blue Screen; Can't write to drive C:

In a school lab we have 21 Dell desktop computers. These
computers have been running well for about two years now.
The computers are running on Windows 98 SE. Recently a
blue screen comes up with a message that the computer is
unable to write to drive c: It appears to happen only
when the computers are idle. You can press any key to get
back to the normal windows view but after the initial blue
screen, the computer becomes unstable and the blue screen
will come up more frequently. The computers are imaged
but there are three different sets of images because there
are three different computer hardware configurations. The
blue screen appears on all three sets of computers. The
one thing in common that the three images have is that I
recently installed Office XP on all of them and that is
about the time when the blue screen started coming up. I
have verified that Office XP has something to do with it
because when I remove Office XP the blue screen does not
come up anymore. I have run a thourough scandisk and
defragmented the drives but it does not help. No one else
seems to have this problem. Please help, I have spent so
much time already trying to resolve this problem.