randomly... random is extremely hard to trouble-shoot.
But you might be able to help.
First, we don't know what OS you're using, but you do.
Windows 98 isn't as stable as say Windows 2000 or XP, and
I'd be more inclined to believe the shutdown would happen
in there.
Second, you may be able to see if you have other
applications running at the same time, or if you're
clicking on something. Look for consistencies. Is virus
checking on? How about your fancy new printer driver?
What if you turn off your scanning software? Is the
system running slow first? Does it shut down, or does it
literally reset like your computer's reset button would
do? Do you find problems on the computer when it starts
again? (i.e. scandisk)
These are just tips on how to find a problem, not
answers, but you might be surprised what you DO find when
you start looking closely. About 50% of the time, when I
have someone do this kind of checking, they narrow it
down to something. Hopefully, you'll be on the good end
of this!
Good luck!
Tom