Laptop shuts down/no BSOD or warnings

I’m not sure what to make of this. I have a 4 y/o HP zv5000t laptop that has:

100GB 4200 rpm HDD
2GB RAM (shows as 1.87GB)
Windows XP Pro SP3

I’ll be working on something and *poof* laptop shuts off. I know I have to replace the fan on it, but I’m trying to put that off until I can get the funding for it. I’ve blown out all of the ports, openings, etc that I could find and I use a Targus HeatDefense pad and am looking into buying one of those Portabook stands.

Why is the laptop just shutting down? What can I do? Any help would be appreciated since I need an answer (if possible) by June 15 (start of summer semester for college). Thank you!