My computer won't shut down properly. Ubuntu 12.04

I have an older computer that I got from work when we upgraded to newer ones.
I don't know the brand, it's kind of a mix and matched machine, but I can get some better info if necessary.

Anyway, I try to shut it down properly from the shut down button in what I call the "start" menu in the upper right hand corner of the screen. However, it just goes to the purple screen with the word "Ubuntu" with the 5 red/white dots below it, and stays there all night.

The only way to shut it down is to unplug it or turn off the switch on the power supply.

Is this a Ubuntu issue or something in the BIOS?

It had Windows XP on it previously, and I don't think it had this problem with Windows, but I'm not certain because I installed Ubuntu on it almost as soon as I got it.

Any help is appreciated. Thanks!