Vista Start-up Problem

I have a computer that runs on a Vista Home Premium and I'm encountering
problems getting the computer to fully load after I boot it up. It'll go
through the process of booting until it gets to the screen where I input a
password to grant entry. But when i do, I get a screen with the "Welcome" and
loading sign for about 1 and half minutes and then the computer screen goes
black. All operations before this seem to work except that the option to
power down from the log on screen efore entering my password doesn't work as
well. I get a screen that says "shutting down" with the load sign and it
doesn't shut down. The computer is an HP with a AMD Phenom processor if that
helps.

thats why i only use INTEL processors, the only processors that should be on
market