"I cannot understand how mine can just "die" without any warning."
Everything that breaks was working fine just before it broke.
There are lots of possible causes. If you know so little about computers, I strongly recommend you to find a good independant technician.
Possible cause of a dead tower are
The power supply
The motherboard (the main circuit board inside)
Faulty add-in board
Other faulty parts.
The ONLY way to find out what pert is faulty is to change them one by one, and see what cures the problem.
It's very likely that all your work is still intact on your hard drive - and you do have it backed up safely somewhere don't you???
It's possible that your system is slow because it hasn't got much RAM if you're running WindowsXP.
You system cannot be 9 years old if the Celeron 2.3GHz was the original processor, as that wasn't introduced until mid 2002.