To reposition a rectangle or elliptical marquee, first drag to create
the selection border, keeping the mouse button depressed. Then hold down
the spacebar and continue to drag. Release the spacebar, but keep the
mouse button depressed, if you need to continue adjusting the selection
border.
Your Photoshop preferences may be corrupted. Try trashing them.
Make sure that your keyboard shortcuts are set so that Spotlight isn't
activated with spacebar. You now have to remove or reassign most of the
OS shortcuts to get Photoshop to behave properly.
I found that on my system (MacBook Pro, OS 10.5.6) that the Spotlight shortcut is Command-Space and even if I were to press this and make Spotlight appear, I'm still able to reposition the marquee as normal.