Snow Leopard not connecting to wifi on Macbook Pro

I reformatted my macbook pro with Snow Leopard installed about half a week ago and it has stopped connecting to our wifi. The icon it shows for wifi is no bars with a '!'. It sees our internet in the menu bar but fails to connect properly. I have tried resetting the PRAM, that does nothing. All other computers in the house can connect to it. I've tried turning the laptop, airport and our house's internet off and on again. None of this has worked. Any ideas?

After returning to a time machine backup, i decided to totally reformat. I did this and on the first time booting into OS X Snow Leopard, it wouldn't connect to the internet. I'm not sure if it's just our home internet playing up or if it's the wifi inside the computer, but whatever it is, i have no idea what i'm supposed to do.

Any advice?