To use this pokemon emerald download on a pc you will need 2 things. 1st thing you will need is the pokemon emerald download rom. A Rom is a gameboy game that is maid to be played on a computer/PC. "The download link is below" But most are not easy to find that are free online because of copyright so the only way I can offer this free is that you must do a free survey. Witch most of the time is just filling in a name and email The 2nd is a Gameboy Advanced emulator. This is basically a Gameboy Advance on your computer there are many out there but the one I recommend is from http://www.accessroms.com/ they offer every emulator you can think of. And offer step by step guide no how to download and get it set up. then you ready to play.
Enjoy!!!