Not 100% clear on the processor issue (the 'missing' pin)... it's likely still lodged in the socket of the mobo, which will cause problems if you're installing to that mobo (hard to tell which you meant, since we're talking about 2 systems), in either case, WOOHOO that means time to upgrade motherboard, and be abit more cautious with the next CPU install
And yes, the motherboard does have several screws that all need to be in-place and 'grounded' to the mount-plate of the case. Try to make sure all screws are down good and making contact with the metallic lining around the screw-holes on the motherboard. I've had situations where a slightly 'un-screwed' screw was making poor connection, basically creating a spark-gap and shorting out the mobo. Beepbeepbeepbeep etc. Unsure in your case, but worth a shot. Check the rear of the motherboard, make sure none of the raised solder-points are contacting any of the bare-metal of the mount plate, as this could also cause a short (very strange if this actually happened....) May be hard to check this if your mobo mounting tray isn't removable... so if you can't check, no worries. Look for slightly un-mounted PCI cards, re-seat your RAM, make sure contacts aren't too dusty, etc. These things can also cause problems. Check for something silly, like an unplugged P4 connector next to your CPU... OMG I have been building comps for over 10 years, and have done this not once, but THREE times! I just get too excited lol. -Drak