HP Pavilion xt858 no beeps screen blank

This computer was booting properly a few days ago. It would bott to windows and then lock up a few minutes later. I powered it on and off so many times, I'm wandering if it was damaged. I get no beeps, the keyboard lights for a second and than goes off. I have tried a seperate pci graphics card and that brought up the HP screen for a second. That was the last I ever saw of it! I have tried booting without the hard drive to get me into DOS. I can't even get into DOS with a boot disk. There is no cursor on the screen. The fan is running on the CPU. The memory is correct. I have reset the bios jumper and reset the CMOS battery. Nothing is working. HP's site is terrible. Any help would be appreciated. Thanks.

When you say you tried booting into DOS without the harddrive, did you unplug the IDE cable from the hard drive or just tell it to boot from floppy in the BIOS? Pull the IDE cable off the Harddrive and see if you can boot by floppy.

I took the power and ide cables off of the cd-roms and the hard drive. I tried to boot to the floppy alone to get into dos. I have no access to bios because the screen has been blank since this started.I have run Norton AV on the hard drive in a seperate machine and the hard drive is fine , no viruses, no errors. I know sometimes the manufacturers have sequences where you unplug the machine for a period of time and then hold the power button down for 30-60 seconds or boot to a certain combination of special keys on the keyboard ( other than F1, F2,F5,F8,F10 or delete). I am wondering if the mobo or cpu has gone bad. I don't have another cpu to test it. I have seen similar problems in forums with no answers. Any guesses?