It is a generally accepted rule that any proprietary built system (Dell, HP, Gateway, etc.) will always use something non-standard to keep you from upgrading pieces and parts. It wouldn't shock me at all to see HP put the mobo screw holes in screwy locations. Buy a cheapo mini-tower, ATX standard and see if it fits. You may find that the motherboard might screw down right, but the CPU HSF may not fit at all due to it being an HP part. You may have to add a standoff or two to the new case as well, but that's easy enough.