First order of business is to check and double check ALL connections, take video card out, clean gold connectors and replace. Look around for anything not plugged in.
Then try and boot up.
Next order of business would be to re-set the bios. If you have the manual or cd with manual, get the info from that. Most of the time its remove all power to the motherboard then remove the coin battery then move the jumper from pins 1 and 2 to pins 2 and 3, leave them a few minutes, put jumper back to pins 1 and 2, put battery back in, add power and boot.
If that fails, best thing to do now is to take out all usb and pci items that you dont absolutely need. Dont take out video card if its needed, but take out all the rest. Also unhook everything that is ide except your boot hard drive and see if it will boot up.
If that fails, try a different keyboard and mouse, if you have extras.
If no go then take out cpu\heatsink\fan. Clean dust\dirt from heatsink and fan, remove thermal paste or pad from cpu, clean with isopropyl alcohol and apply new thermal paste and re-seat all.
If no go its time to take everything out of the case and let us know when you have done that.
Also, can i asked how you checked the power supply? There are different rails (voltage lines) and each and every line has to be checked. You need a multimeter and know-how to do this. You can get a tutorial using google or go to techrepublic, join free and find the tutorial for troubleshooting power supplies.
Good advice + great people = tek-tips