Ntoskrnl missing + blue screen

I have a Dell Dimension, XP SP2, 2g ram. Raid hardrive combo 2 drives of 180GB each. One day out of the blue I restart my pc and get an error ntoskrnl.exe missing. Booting in safe mode brings same thing. I then tried to reboot of the Windows XP disc and I get the blue screen of death just after all the files are loaded and just before the setup should begin. I tried a 2nd XP disc and got the same. I ran the bios hardware check which takes over 24 hours and it turned up no hardware problems. I disconnected floppy and 3rd harddrive and made no difference. I then removed the 2 raid array drives and tried to load a fresh copy of XP onto the 3rd hard drive and I get the blue screen again. I did the same with a vista disc and it freezes half 20% into file transfer. So basically I've tried a combination of different HD's and Windows discs and can't get it to boot. Any ideas what could be causing this? I am running out of ideas here!