Well if you have a stock A500 with just 1MB of RAM its a bit hard to do. I used to do it all the time with my A500. The best way is to get a serial null modem link going and use the amiga2PC network utility. I dont remember the exact name but I'm sure someone here does. What you need to do is set it up to read off the pc floppy drive and just use "tsgui" from aminet to write the .adf file back to a real amiga disk.
The other way ony if you have a more powerfull A500 with more then 1MB of ram is simular. I took a null modem link and used jrcomm (a terminal program) and used teraterm on the PC to send the adf to ram on the amiga 500, then used TSGUI again.
