Unexpected shutdown problem!

[eagle_y2j]

I m having 845GVSR mot. board with 256 mb RAM & OS is Win XP Pro Sp2.
My problem is that my PC is getting shutdown without any good reason and when i try to start it again both the indicators of cabinet are glowing
i.e {power & HDD} I can hear my fan runing but CPU does'nt BOOt up again till I press Keyboard wire inside its port tightly but after another restart a min later problem is back................. Help me out of this plz.......

[alib_i]

must be a loose hardware component ...
check for loose ram, and anything on pci slots ...

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alibi