Are these branded computers like Dell or HP or did you build them yourself?
How did you update them? Who is stated as the driver provider in Windows Device Manager?
Are you recording with an external mic plugged into the pink mic port, or are you recording something else?
Have you tried
http://manual.audacityteam.org/o/man/faq_recording_troubleshooting.html#flatline
and
Audacity Manual ?
Gale