i've had it for a while now and i was gonna put it in my laptop but since it's old and the one its got now its working fine i figured id put it in my "fat" ps3 160gb but can anyone tell me how to do it do i need to download something or transfer something from the other one or can i just take the old one out and slide the new one in ?

First, any save data on your old hard rive you'll want to put on a flash drive. Then use a flat head screw driver to remover the side panel revealing the hard rive. Use a small phillips head screwdriver to remove the hard rive from the little tray it is in. Screw in the the new hard rive and and start up your playstation. You should get message saying that their is now software on the the hard rive. Go online to sony's website and download the latest software on to a flash drive. Plug the flash drive into your playstation and let it download the data. Once this is finished you can put all of your save data back on the playstation.. Finally go to the create psn account button and click on the retrieve account button and enter your password. Doing this should also retrieve all your trophies.

It is not really worth the hassle.

You'll probably mess up your PS3, but you'll certainly void any warranty when you do this. I don't like PS3 anyways, so I hope you burn out your motherboard or CPU.