I'd like to do a small project with an Arduino inside my PC case, but I'm not sure where I can find 5vdc while the pc is turned off. Can I just connect it to internal usb 5v pins or can I use 5v from a molex connector? Do they even supply power when the computer is off?
Most PC power supplies have a 5v "always on" circuit... everything else will turn off when the PC is off.
In a standard ATX this is normally the purple wire. This can only supply about 500mA but might be enough depending on your requirements. It also has to power the "always on" components of the PC.
Check out your particular power supply... usually there is a label showing what each of the wires are, and what their maximum ratings are.
you can use internal USB connector
As Juraj suggests, on a desktop PC the USB ports are usually powered by the standby 5 V supply to enable wake on keypress or mouse and possibly (but not always) start on keypress. Not generally on a laptop however.
The disk drive ("Molex" or SATA) connectors however are powered down.
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