Connecting WRT120N and DWL-G700AP

I am currently trying to get my access point DWL-G700AP to connect to my existing network. I would like to extend the range of my network into a garage/shop area so I have net access while doing projects. I am running all older machines down there with Windows XP SP2 or greater.

The router I am using for the existing network is a WRT120N. All of the computers running Windows Vista have no problems connecting to it. I have several machines running Windows XP and only one of them connects to the existing network. I guess I was hoping by getting the access point working I would in turn get the rest of my machines back online.

If anyone could help me with step by step instructions or help it would be great.
I have some limited understanding of networking, but I guess not enough to accomplish this task. Thanks in advance.