Computer freezing up

We have Windows 98 as our operating system and are running
Microsoft Word for Windows 2002. Whenever I leave my
computer for a 15 to 20 minute period and return, no
matter what program I am in, my computer is completely
frozen and I have to shut down at the main switch and
start over again. Our office is on a network.
Can anyone please give me a suggestion as to what I might
try. Seems everyone is stumped by this strange behavior of
my computer. This happens anywhere from 3 to 7 times a day.
Thank you very much in advance.