How do i set up my laptop to my G Router

Hello, guys!
I need it to reinstall my hole operational system (Windows XP) because some virus came up....
Well the problem is, im not really familiar setting up network, wirelessly and well it worked before it was connected before I reinstalled my O.S. I need help how to set up my wireless network on my laptop( DELL Inspiron E1505) with my Wireless G router(My Essentials, the brand). this G Router Is directly connected to the internet, and my computer is connected to this G router by cable. I need my laptop to connect to this G Router wirelessly.

I know this is to much to read but i explained it the best i could, SORRY

Thank you