Ah, the dreaded Via chipset!

First, make sure you have the latest chipset drivers installed (the Hyperions from www.viaarena.com ).

Second, you may have to set the AGP slot as low as 1X to get full stability.

If you have ACPI enabled, the video card may be sharing its IRQ with one or more other devices. That is guaranteed to give you lock-ups during 3-D acceleration.
The downside being that if you disable ACPI, you may have to re-install WinXP.

Hopefully one or more of these will help. Good luck.