Dell Mih61r Mb Front Panel Pinout 'link'
Most Dell systems of this era, including the Inspiron 620, do not have a dedicated reset switch header. If your case has a reset cable, it will likely remain unused.
The MIH61R is an LGA1155 motherboard (supporting Sandy Bridge and Ivy Bridge CPUs like the i5-2400, i7-2600, or i7-3770). It is old, but capable for a budget retro gaming PC or home server. dell mih61r mb front panel pinout
This is a classic symptom of a short on the Power SW pins. You probably have something metal touching pins 2 or 4, or you connected the Power LED to those pins by mistake. Unplug all wires and start over. Most Dell systems of this era, including the