Warning: I’ve never seen a Dell Optiplex GX260, I don’t work for Dell, I work for one of their competitors.
Desktops are typically built with a chassis and two side panels. If your power supply seems to be held in place by invisible screws it could be that the working end of these screws is hidden under one of the side panels. See if you need to remove the side panel to get at them.
Out of the normal brands of PCs, I have found that Dells are generally friendly to work on. I have done a bit of dell contract work and HP.
I will never have anything to do with HP ever again.