First of all, thanks for your help.
Ran Malwarebytes, nothing found. Installed Superantispyware. On confirming the Superantispyware language, computer video went to horizontal lines and audio buzzed for about 5 seconds, then system rebooted.
I reformatted and installed Windows 7 x86 RC 7100. Enabled and logged in as Administrator. Installed a RaLink 26xx Wireless PCI LAN card. Installed Avast Home. Reboot without boot scan. Added serial. Within a couple of minutes, system froze as before.
Went into safemode, deselected all options under Programs Change/Uninstall, except Language (required). Rebooted. No crashes. Selected Web protection under Programs Change/Uninstall. Reboot. System froze after log in.
I made no other configuration changes, installed no other software, including cancelling the the prompted Flash player. It seems I can only run Avast without the additional enhancements selected.