If building a breadboard Arduino and powering it with a computer power supply that outputs 5v, do I still need a voltage regulator? Is there such thing as a 5v to 5v regulator?
You can connect it directly. I would, however, include a 1 amp fast blow fuse in series with the 5V line. Computer power supplies can put out 25 or more amperes at 5 volts and something less than a DEAD short (like a thin wire cliplead short) will result in a spectacular cloud of smoke followed by flames.
You may be safer to use a wall wart or build your own supply.