Assuming you did everything in my first post (if not start there again)... then try this..
Next, remove all hardware except for what is needed (Ex. Video and HD). remove everything else for now.
bad hardware/mis-seated can cause the prob your having. check the pins for the agp, pci & isa slots on the mobo, make sure they are all in good shape. If you have other video card or hard drive, try putting those in it, just to see if it will boot.
if it still does not boot with known good hardware:
Reset the bios completely. You can do this a couple of ways.
1. take battery out for an hour or two
2. See MB manual, it may have a jumper that you can switch to clear the bios.
If that doesnt help, you are getting very close to running out of options. I would find the motherboard info and find the right flash for bios and flash it. This probably seems wierd, but I have seen a case where as soon as you plugged a machine in, it would power up (yes without pushing power button), then hang on the POST. was wierd as hell, but the bios got corrupt somehow (would not even reset itself) and reflashing it fixed it. After i reflashed it when i plugged it in it no longer came on until power was pushed and booted normally.
Good luck!
for a plane ticket, ill gladly visit canada to fix it for free