If you could answer a few questions, maybe we can figure out what's going on:

1) HP Model Laptop?
2) New or Used?
3) Came with Vista already pre-installed?
4) Was it plugged into the wall outlet when it shutdown suddenly?
5) Are you getting the date and time error every time you start?
6) Any updates made to the laptop just prior to the problem starting (Windows Updates, software install, driver update)?

One thing you can try is seeing if you can get Vista stable in safe mode. If you are not familiar, when you first start the laptop (the DOS like screen), start pressing the F8 key before the screen goes black and you see the Vista startup. This will bring up a menu screen in which you can pick "Safe Mode". Safe mode will boot up Vista with just the bare minimum (mostly no drivers or applications that would normally run when Vista starts). Because the video drivers aren't loaded, things will look a little weird and larger than normal. Log in and see if it does the same thing.

Let us know how that test goes and the answers to the questions.

Michael