Windows Vista automatically restarts if your PC encounters an error that causes it to crash. This doesn't matter if it happens once in a blue moon, but if you have a serious problem that happens each time you boot - your PC will constantly reset itself.
The easiest way to solve this problem is to disable automatic restart within the Vista control panel. This won't solve the problem causing the crash, but it will provide some on-screen information that will help you diagnose the error.
To disable automatic restart, press WINDOWS KEY + PAUSE/BREAK to load the System Properties Dialog (you can also access this via the Control Panel > System). Then click on "Advanced Settings":
Then, select Settings in the "Startup and Recovery" area:
This will load the Startup and Recovery window, allowing you to untick the Automatically Restart box and then click OK:
Next time your have a crash in Windows Vista, your PC will not automatically reset and you will be able to view the error message.