I've been using Win XP with AMD Sempron 2800+ processor, 512 MB RAM & 200 GB harddrive sucessfully for the last year. Now all of a sudden when trying to power up, nothing appears on monitor and computer shuts down after only 15 seconds. I unplugged the monitor and the computer stayed on for a few seconds more. I thought possibly the power supply was weak, so I installed a new one only to have the same problem. Any suggestions?

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