If so disabling the “restart on system failure” feature may permit the exact cause to be isolated: Right-click on My Computer, click Properties, click the Advanced tab. Under “Startup & Recovery,” click Settings. Under “System Failure,” uncheck the box in front of “Automatically restart.”
I work on a rule of thumb that a modern machine with just the basics (1 x processor, 1 x HDD, 1 x CD unit, 1x floppy) needs a 300 Watt PSU. ANY additions such as a graphics card with fan, another CD unit (especially a writer), another HDD, non-self-powered USB equipment etc., allow another 25 Watts each.
So, a machine with 2 x CD/DVD units, 2 x HDD, 1 x cooled graphics card, 1 x USB item would have a 400 Watt PSU. (300 W basic + 4 x extra items @ 25 W each)
This is very empirical but it does allow a good reserve of power should it be necessary. Upgrading a PSU from 300 to 400 Watt doesn't cost a lot.
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