Well, this is a tough one - in point of fact, it's probably not your CPU
causing the crash if it's a hardware problem at all. More likely would be
something like Excel using a portion of RAM that doesn't otherwise get used
and that could be causing a crash. A way to test the memory is to get a copy
of MemTest86+ from this site:
http://www.memtest.org/
They've got versions available for creating a bootable 3.5" floppy, and even
for a USB device, and others. Create a bootable media and use it to run the
tests to see if they indicate any memory errors. The longer you can let it
run, the better - and it'll run until you tell it to quit. Might be a good
thing to allow it to run overnight if you can.
Barring hardware issues, if it's an Excel problem, you can always try a
repair installation of Office/Excel. If you have 2007, it even comes with
diagnostics to attempt to tell if any of the Office/Excel system files have
problems that would indicate a need to do a repair install. You should also
make sure that your copy of Office/Excel has all current updates applied.
I've seen problems that caused Excel or Word to crash (not the whole system,
just the application) that got cured with an update from the update site.