I've had a lot of people ask me this so I thought I'd add a FAQ on how to do this.
What you need :
DVD reader(for making iso's)
DVD writer(for copying iso's to work on the PS2)
PS2 with a way to play copied discs(to play the game )
DVD Decrypter http://www.afterdawn.com/software/vi..._decrypter.cfm
PS2 game(if making an iso)
PS2 iso(if making a copy)
Making an iso from a game disc :
After installing DVD Decrypter start it up. Go to Mode and go right to the bottom where it says ISO> and select read. You can pick where your iso goes to. I usually export it to my desktop for easy finding. After you pick where you want your file to go hit the icon of a DVD with an arrow to a Hardrive and your disc will begin writing to your computer.
Making a game from an iso :
Instead of selecting read in mode select write. You can then find which file you want to make a copy of. If there is a MDS file select that instead of the iso. Otherwise just select the iso. There may be a range of other file types(.nrg for example), just treat them as an iso. Select the DVD to HDD icon instead with a blank DVD in the writer and your good to go.
Hope this helps. If it could get a sticky that would be cool