Aspire 1644 WLMi, XP Pro, frequent reboots.

Hello. Laptop is Aspire 1644 WLMi, XP Pro, 1 Gig RAM. Increasingly frequent spontaneous rebooting. HD fault diagnosed, poss. from going above 100 C. Replaced HD and cleaned cooling fan. Attempted fresh install XP Pro; repeatedly sticks at 34 mins and then reboots. Halted reboot/restart of install by removing install CD and, via DOS prompt, used setupapi.log to diagnose possible hardware failure. Removed audio .inf file from Windows folder. Rebooted. Install completed, but rebooting occurred in default mode (no sound, of course). Then booted into Safe Mode with Networking. Laptop stable in Safe Mode and installed service packs ok - also stable in BIOS and DOS - suggesting that HD and power supply are ok. Next, reformatted HD and attempted fresh install of Windows 7 Pro 32-bit; got as far as rotating-coloured-stars screen, then it rebooted itself.
So, my question is: what next? It's looking most like a h/w problem - and I need to narrow it down. How do I do that? And if it turns out that the audio chip has failed, what are my options? Any ideas gratefully received.