You probably cannot use the tranformer in a "regular" linear power supply... Most (all?) computer power supplies are switching designs. There is an oscillator and the transformers run at much higher frequencies (higher than the 50/60 Hz line frequency).
The problem, however, is that the transformers on ATX PSU read 0 ohms on the primary and secondary. How is it so?
That's the DC resistance. Transformers run off AC. With no load, the coils are like an inductor with an impedance (ohms) that's minimum at DC and increases with frequency.
What is maximum current an arduino can pull? 1500mA ?
Not that much... It should be less than 500mA depending on what you have connected. The ATmega chip itself can pull a maximum of 200mA if you have the I/O pins "loaded down", and I'm not sure what the USB chip takes.