Out of ideas, Windows startup is halting

I am having a very puzzling boot problem. This problem is occurring
on a homebuilt Windows Vista system, 2 months old, all new components.


How it failed: while I was moving files in Explorer. Suddenly froze.
Problem started upon reboot after that. The system had worked for 2
months (since purchase) just fine.

Reboot sequence: boot process proceeds normally, moves through the
Windows progress meter on the bottom (progress displays normally),
then moves to next screen where video change causes LCD monitor to
refresh. Everything halts, black screen.

Here's an interesting fact: when boot reaches this point, Cntrl-Alt-
Delete doesn't work. The power & reset buttons on the PC case do not
work. Both of them work fine up until this point. I have to cut
power at the back of the power supply, or at the power strip to turn
the computer off. The power button DOES work to turn it on.

Software diagnostics: it boots this SAME way with:

Local Widows vista install (full, safe mode, safe mode with command-
line)
Windows Vista install DVD
BartPE XP w / diagnostics disk (requires no HD to be present to load)
Kubuntu Linux install DVD (requiring no HD to be present to load)

Hardware diagnostics, all with no success:
sent PCI-express card back under warranty, received new one
Installed normal PCI video card
Replaced LCD monitor with 15" Optiquest monitor.
Tried digital video output - analog converter instead of analog video
output.
Memory is 2 gigs, removed each to see if it would load with just 1
gig, did several memory scans using DOS-based boot memory scans,
passed.
Unplugged all unnecessary HD, USB, & peripherals to lower power load.
Power supply is 600 watt.
Voltages in BIOS all look fine.

BIOS works fine, DOS programs (the memery check programs) work fine.
I can't think of anything else to test. I've run out of ideas, and
I'm hoping for other peoples insight on this.