The HP E-PC is (as far as I can tell) a proprietary format. It has an on-board PSU. There are two types of PSU's for it - one is a 19V barrel connector with a power brick very similar (if not identical) to the HP laptop power cords. The other is an 8-pin connector with the 12V, 5V and GND supplied at different pins. Check to see which you have. There are threads about this scattered around this forum (I think some schematics for that 8-pin connector too). If you have the barrel connector, you could just use the Carnetix 1900 and be done.