XP Splash screen then goes to black screen, no video. Help!

I am working on a Dell Dimension 5150.  When I boot up the machine it goes thru POST and brings up the loading Windows XP screen, and after that loads the screen goes black with the cursor in the middle of the screen which won't move. I've left it sit there for along time (like up to an hour). Video is integrated. Here is what I've tried thus far..

- Booted into Safemode with same results (although I do see the Safemode msg displayed in the corners, but it just sits at black screen with cursor in middle.

- Did F8 and booted in VGA mode, still no luck

- I've tried 2 different new video cards, no luck. Both cards have dual monitor mode, so hooked up both monitors in case it was showing on the #2 monitor, no luck.

- I've unplugged the power supply and let sit for 5 minutes, no luck.

- I've tried F1 & CTRL ALT DEL and any other key strokes, no luck.

- From WinXP install cd, from C:\WINDOWS prompt, I've run CHKDSK /R and CHKDSK C: /R, still no luck.

I'm not sure what else to do here. Once the screen goes black and can still hear the HD working, so I wouldn't think it's a bad HD, but I could be wrong.  I haven't run a repair install yet.   Can anyone help me with more ideas?  I've been searching online for the last 2 nights and nothing seems to work for me.