My PC is screwing me HELP !!!!

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hey folks i ve got a pentiun 4 1.5 gig processor on a 845 intel chipset..i bought it in dec 2001 ...so the motherboard is one of the earlier version of 845 M/B..it is havin 256 MB of SD RAM and a 64 MB Geforce2MX400 AGP ..it dont have onboard video...have a 80 gig HDD
i m running microsoft windows XP Service Pack 2 release on it..the problem is when ever i boot my PC the screen becomes blank and the monitor switches in low power mode ( i.e. the diaplay disappears)...power button doesnt works neither the reset button works...when i reboot the machine using the UPS....it stays up for some more time but again samething happnes and the display goes blank...
if you are feeling lucky enough the system stays somewhat longer (say half an hour) in up condiation but after some time it again does the same thing
i ve reinstalled the windows...changed the AGP even i ve changed the CPU FAN...tried to run the system with single ram module...but simply i am not able to diagnose the problem also i ve changed the drivers for the AGP..but noting worked
the installation process runs fine even in safe mode the thig does properly

Help me out folks i am despo

bro..seems u have a realy strange problem......may be ur mobo has developed problems ..(seems the only reason)...if possible....chk with other mobo..as the guarentte is gone.....so u cant replace it ....also chk if the ventilation is proper or not ..use intel's active monitor to chk ur mobo's temp also

dude...the same thing happened with me long time back...my hdd was screwed then...i got a replacement n everuthing worked fine...try gettin replacement if in warranty

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@ Saroman
thats what i thought...but at times it works fine..anyways will check it installing Win98

@chinmay
i just got this new HDD eight months ago...it works fine on other machines as well...tell me something else