Had same sort of prob with boys pc,so changed psu but to no avail.All this after checking memory/graphics card e.t.c.As i had a spare mainboard, i decided to replace this and also reformat at the same time and now all works fine. In coclusion,not sure if it was the o.s or the board,but my guess would be the board.As yours was after a power cut or storm,if the psu change don't work then i would change the board,bearing in mind you should be prepared to reformat when doing this.
been thinking.the surge could have knocked your bios for six,so you could check this out or set it to default if not too sure.Also you could try reinstalling the motherboard drivers via it's cd if you have one. You never know,it might save a few bob.
the only google i can find explains that the 3.3v power supply has failed but the 5v has not. too heavy going for me,but seth may be right with the battery option. since they are cheap, might be worth a go. click here
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