Brian Rabbit
As you already been advised, it's worth replacing the power supply, with a 'Known' working unit.
Unfortunately, while a person can make an informed diagnosis, sometimes computer troubleshooting, due to the complexity of components, is diagnostics by substitution.
If your Inspiron 580 is a Mini Tower and not the Slimline version.
The Dell Inspiron 580 Mini Tower can use most generic standard ATX power supply units, with either a 24-pin or 20+4-pin main motherboard power connector, with or without the on/off switch.
You should be able to buy a compatible power supply from either a local or online computer store.
Note: You require a power supply with four SATA power connectors.
Bev.