I have a HP d4100y that dropped dead last night. I tested the power suppy with a multimeter and did not get any reading until I hit the 9 pin (+5 V standby). I am assuming that it is toast. Being very new at this, how do I choose a new power supply that will work in my system. Currently there is a PS 6401-08HT (HP 5187-6134 OEM) power supply present. 400 Watt unit. I have also not found any dimensions in any of the product descriptions. Any help would be greatly appreciated.