Hi all:
I also got problems when loading a sketch for the first time. I tried everything but it was impossible. (Pressing the reset button before, during , after the upload message appeared didn't help me).
I use Windows XP.
In may case, I solve the problem by plugging the Leonardo to a different USB port on my Laptop. The drivers reinstalled themselves, a new virtual port was created and, after this, everything works fine. As I was using before a Mega on the first port, I thing something was mixed together.