I have a pretty GOOD computer that is about 1.5 years old.  I have windows XP.  Everything with my computer works pretty well except for one problem.  I have hi-speed internet access and after about 30-45 minutes of using the internet my computer will suddenly shut itself off and RESTART.  It restarts fine and goes BACK to working fine, until I use the internet for another 30-45 minutes and then it shuts down again.  Does anyone know what the problem is? Thanks for your help.freddiezee..... Have you checked to be sure you don't have some nasty guests?
Can you check your CPU temperature and see if perhaps it's a heat issue that causing the shutdown?

dl65  I don't think it's the fan, since the comp. only shuts down if on the net and not in any other program.  What do you mean by nasty guests? How do I take care of that?also, how do I check the CPU temp?What is the computer or motherboard make/model? There may be a utility to check the temps.

He meant scan for spyware, adware and viruses. What programs do you have in place and being used regularly for this?