My laptop won't boot, doesn't POST and am trying to see if it's terminal or prohibitively repairable. When powering up, it whirs, the fan starts and a few seconds later ( between 3 & 10) it shuts down never reaching the onscreen COMPAQ logo. Happened after a restart when I couldn't get a USB stick recognised and it never breathed life again

It beeped on one occasion at the beginning of 'the troubles' but I didn't note what the sequence was and it hasn't squeeked at me since

Have tried a new battery, no battery with AC only.
Have tried swapping RAM, no RAM, 1 Stick of RAM
Have tried a new hard drive, freshly formatted
Even tried booting from an XP disc - nothing works

I get nothing on the display so have no messages to report. All seemed fine until the last re-start. Am guessing it could be a failed CPU or motherboard which wouldn't make for an economic repair. Any suggestions?

Compaq Presario 2178EA (AMD)
Windows XP SP2
1 Gb RAM (2 * 512 Crucial)
20 Gb HDD

Thanks in advance