I got it going now. I did a clean reinstall from the downloaded Vista iso, and it no longer froze up.
But when I loaded the graphics driver for the GeForce, it would unload it on its own, and device manager had a message that Windows disabled the device because there were problems. Tried various things, could not get a driver loaded. I finally replaced the graphics card, now all is fine. Kudos to beret21 for suggesting that early on in this thread.
And the system is much peppier with the clean install than it was with the bloated factory default install, as gregrocker always says. Thanks to all.
By the way, I did not load any drivers from the HP website for this Pavilion a6250t. Device manager was not showing any issues. Do I need to load the chipset drivers, etc? It's running so well, I tend to want to leave it be.