I have a doozy and am hoping that someone has seen this before:

We have a customer's Sony vaio computer. It has Windows XP Home SP2, integrated video, integrated ethernet, 2 sticks 256MB DDR RAM; only internal card is PCI modem. It's using PS/2 mouse and keyboard. When booting normally, it gets to the Windows XP logo screen with the scrolling bar at the bottom. That disappears and the screen goes black and it never displays the logon screen. When booting into safe mode, it hangs at agp440.sys. Here's what I've tried so far:

1. Replaced RAM with known working RAM.
2. Unplugged every unnecessary item from mobo to rule out hardware problems (2 optical drives, floppy drive -also disabled controller and floppy seek in bios when floppy unhooked), removed pci modem)
3. booted to recovery console and ran chkdsk /r and it did find and repair some system file problems, but did not solve problem
4. booted to recovery console and ran listsvc and disabled agp440.sys as per Microsoft KB #324764... after doing that it now hangs at mup.sys when booting in safe mode... have not disabled any other services
5. Early on, tried last known good configuration when booting
6. tried different ps/2 keyboard and mouse... also tried usb keyboard and mouse

This system did not come with recovery cd(s) or XP cd, so haven't tried repair install.
Would love to hear other suggestions. Thanks in advance.