computer restarts itself automatically

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I am having a problem with windows xp restarting itself automatically. the computer in question has never been connected to the internet so I can rule out viruses. I have also tried doing a complete re-install of windows and completely wiped the hard drive clean so I am pretty certain it is not a software issue. How do I diagnose the problem if it is a hardware issue? Also, sometimes it can't load back in to windows. Sometimes it does without any problems but many times it gets stuck on the brand icon at the begining and never loads in to windows. when that happens the only way to get around it is to turn off the power alltogether. Thanks!!!

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Well as it turns out I found out the problem.... the RAM was bad. I switched it out and haven't had a problem since... hope this helps somebody else who might be having the same problem