New PC Can't Find Wireless Networks

Hello,
I just bought a Dell Inspiron PC, and I have it all set up except for the internet. I have a wireless connection in my house, and every other computer we have (a lot of them) is able to connect just fine. I am in range of the connection, as the computer is right next to a range expander, and this laptop which connects just fine.
I have tried everything I can to connect, but through a bit of research I found that my desktop may be unable to use a wireless connection without a certain antenna or something of the sort. I cannot hardwire my computer to the connection because I am a few floors away from the main modem and router.

Could someone please tell me either a way to connect my computer, or what I need to do to open the possibility of using the wireless?

Sean