Self-powered USB device isolation
I have used the ADuM4160 for isolating full-speed USB.  It doesn't work for high-speed.  This was used in a case where the device being connected to could produce kV-sized transients.  This shouldn't be needed just because of ground issues though.

PC (I assume you mean ATX) power supply outputs are generally not connected to earth ground.  Neither is the USB ground on a PC (since it is tied to the power supply ground).  So, there should be no ground loop.  

Note that the different outputs (+12V, +5V etc) on a PC power supply do use the same ground.

If both supplies were grounded instead of floating, they would be grounded to the same ground, so there still wouldn't be a ground loop.  There also wouldn't be a loop if one of the supplies was floating and the other was grounded.  

The only way to create a loop is to have two supplies which are both tied together but one uses a virtual ground which is different from the real ground; for example, if you had a single PC power supply and wanted to generate +7V as 12V-5V, then the virtual ground would be at 5V relative to supply ground for that circuit.  If you then tried to power something in that circuit from +3V as 3V-0V from the same supply, you would have a loop.