Mouse clicks on moving-lockup

I have about 10 computers which all of a sudden have started hanging up
and making a clicking noise when moving the mouse. I have 50 more with
the same configuration which, so far, do not have the problem. They have
ATI video cards, 300 PII, 64 Mb RAM.

I was told that this was caused by the video settings being too high.
They have not been changed. Most are at 24bit–nothing higher.

Any ideas? TIA,

Susan Kennedy