I had this problem at first, where my game would crash as soon as I loaded a new game, and my computer would automatically reboot. Here are my current specs:
AMD 1400 768 DDR RAM Geforce Ti4200
I run Halflife 2 at 800x600 with advanced settings to medium, HL2 runs perfectly smooth.
What you need to do is simple; before you initially ran Halflife 2 you were asked to upgrade your video card drivers. All you need to do is roll back this driver. Go to "My computer", and right click> properties. Now click "Device manager". Then, go to your video card tab, and double click it. At the driver tab, click "Roll back driver". Once done, restart your computer, and Halflife 2 will be running without any crashes.
Took some time to figure it out, but this seemed to fix it completely.
can gforce ti400 (is it this?) be compared with ati radeon 9250?