I cannot get airport express to join my existing wireless network.

I've seen this question asked in other areas but no solution that has worked for me. My network is PC based machines running Windows 7. AT&T Uverse is the ISP with their modem. All I'm trying to accomplish is to stream music to the express from iTunes via my existing wi-fi. I've tried both the automated and manual options in the airport utility. After setting up, it goes through the normal configuration thing and says it is restarting and then cannot locate the device. After a reset (I've tried all three levels multiple times), the utility can find the device again and nothing changes. It stays set to make its own network. From there, I can be successful at switching from my network to the apple network the utility creates and the music will stream. Problem is that I lose my regular network at that point and have no internet. A friend brought over a mac and we tried to do it from there in case it was a PC issue but got exactly the same results. Over time, I have introduced many laptops, tablets, phones, etc. into my current network with no issues so it makes me skeptical that it is a firewall or something in my network preventing it, but I'm at a loss. I did also make sure that the security matched. Surely someone has experienced and solved this. Oh, my express is an older model g. Thank you in advance for your advice!