XP computer randomly turning off.

[1million2012]

I have a rather old computer and it is running windows xp. My computer will randomly just shut down (goes into a kind of standby mood, where the screen goes off and I can't do anything) this happens when I am just browsing the internet or occasionally when I plug something into the USB ports. I this an over heating problem? software problem or a power supply problem?   

Thank you.

[Allan]

Need to know if it's actually standby, the screen going into power saver mode, or really a shutdown.

1) Check settings in power options
2) Boot to safe mode and see what happens