Could be manny thing.
Have you installed any new hardware or software recently?
if so uninstal software or remove hardware see if the prob persists.
when the system crashes go into bios.
to do this hit the delete key or f1 or f2 whilst the ram is counting if you
are not shure when this is just keep hitting the keys untill bios oe cmos
comes up. most pcs should have a pc health tab if not just check them all out
untill you find one that displays the cpu temperature over 60 is getting a
bit warm. If so consider having the cpu re-seated with a new fan particularly
if it is an AMD.
when your system has booted up do you recieve the message that "your system
has recovered from a serious error" If so select "send report". Be shure you
are connected to the net. Explorer should open with a reply .Microsoft often
have the answer.
When your system fails windows usually will write aa report.to view these
reports go to...."control panel" "administrative tools" then "event viewer"
in the application and system tab you will see events and errors listed in
date and time order. Look to see if there is a recent entry to the crash
double click on the error
under the description you will see a report such as this..................
"The Network Address Translator (NAT) was unable to request an operation of
the kernel-mode translation module. This may indicate misconfiguration,
insufficient resources, or an internal error. The data is the error code.
For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp."
and at the bottom you will see a link to microsoft for more info on the
problem.