It sounds like defective memory. You should run memtest from the boot prompt of a Knoppix CD or swap in some known good memory. If you still have the problem after verifying the memory, post back.

hi buggz thank for the reply to the post but it wont let the system run atall as soon i switch it on it doesnt let me go into the boot up log it just switches the system off, i think it mite be the heat sink but im not sure.

any other suggestions.

Have you run through the steps in the My PC Won't Start sticky at the top of the forum? I do't think the heat sink would prevent the PC from booting unless it has already caused the processor to overheat and damaged it. Try stripping the system down to the essentials (Motherboard, processor, 1 stick of memory, display card, and power supply and then see if it will post. This will help eliminate other things like bad drives or other add-in cards. Also disconnect any external devices except the monitor and keyboard/mouse. Let me know how that works out.

You probably have failed caps on the mobo per info on Dell forum: http://forums.us.dell.com/supportforums/board/message?board.id=oplex_bios&message.id=8930

hey bugz my gx270 will install win7 but xp install will give BSOD i tried to use another ram stick that i know works but still the same BSOD i also used Seagate hard drive instead of the Hitachi Deskstar and still the same BSOD id appreciate any ideas

Checkt the capacitors on the mobo and psu
Dell has had a recall on all GX270's and 280's due to capacitors failure

BTW if it's capacitors, I do rework on these boads replacing Faulty capacitors, I however live in europe. Dell charges you somwhere like 300 US for a replacement board. I charge around €60 excluding shipping and you get a tested / working board back.