Think I am having a similar issue....
here is my situation.
last week i went to the local radio shack and picked up a uno board, pretty sure its the non R3 version, but has a 328 chip.
loaded the software onto my computer (1.0 is what it claims it is) and everything works.
loaded several scetches, modifies some etc. Great.
I wanted a second unit to play with, as the uno is has been delegated to a specific project already and my work partner wont give it back, I went back and bought a MEGA 2560 R3. not only was it the only one they had, but now i can make him squirm with envy cause mine is bigger.....he deservs it for comondering my uno.
However, I cannot make it work.
I did select this board in the arduino program.
I get ann error message once its plugged in, saying that new hardware is installed and i need to load software bla bla bla....
But cant seem to find the drivers for it.
tried to download the zip file from adruinos site that should have the propper dirver in it, but i get a corrupted file error message. So what now, this is quite frustrating.