I don't know that a computer would shut down from insuffecient RAM. Defective RAM perhaps. But let's come back to that issue later, as there are more probable causes to your issue.
Overheating? Yes, sure could be. One clue that you often get (but not always) is the computer will slow down just as it is overheating, and just before it shuts down.
You would do well to follow the advice in the following tutorial: http://www.bleepingcomputer.com/tutorials/cleaning-the-inside-of-your-pc/
Also please tell us if you have overclocked your CPU.
Usually when XP automatically reboots it is because Windows recieved an error. We need to make a change to a setting on your computer so we can find out exactly what the error is.
Go to:Start > right click My Computer > Properties > Advanced > Startup and Recovery Settings
and uncheck Automatically Restart
. Now when Windows receives an error it will show you a blue screen.
Now when you receive a blue screen the computer will pause on it and you will have to reboot manually.
The 'Blue Screen of Death' (or BSOD) is refered to by Microsoft as a Stop Message. They can occur for many reasons. In order to diagnose it's cause we need as much info as you can give about the stop message. Please write it down and post it back here the next time it happens. In particular, it helps to have the somewhat cryptic series of numbers about half way down the blue screen that have this sort of format:
0x0000008E (Followed by four other sets of numbers in parenthesis.)
It is also helpful to know the answers to these questions:
- What are you doing when this happens (are you on line, starting the computer, etc).
- Does it always happen at the same time?
- Is it always the same message, or is it a different set of numbers each time?
- Have you added any hardware or software lately?
- Are you experiencing any other buggy behavior from this computer (such as it runs slower than it used to, etc)?
Please complete all the steps above and report back with answers to all the questions above.