Compaq Presario 2100 Boot Up Problem

My Compaq Presario 2100 runs Windows XP. I am unsure of any memory other than the fact that it has a 40 GB hard drive. I also do not know any other specs about it, as I cannot get it to run long enough to find out.

I can turn the computer on and log in, but it will not stay on very long. Sometimes it shuts down as it is loading the desktop. Other times it shuts down as soon as I click on an icon to open a program. I can ALWAYS log on and at least get to the desktop, though. This is very annoying, especially since I have a ton of school work on it that I can no longer access! Any help that can be offered would be greatly appreciated!

 

I can boot in safe mode, but it does the same thing. It will run after loading the desktop for only a few minutes, but then shuts down without warning. I have blown the air vents out, but that has not helped. I have also tried taking the battery out and running only on AC power, but it won't even turn on then. Thanks for the help so far.

 

Unfortunately I don't know a whole lot about computers, so you will have to tell me how to boot to the BIOS. Thanks.