I can use the printer with my lap top that is wired to PC
Do you mean that the printer is wired to the PC or that the laptop is wired to the network?
Here are some more questions:
If you connect your laptop running XP to the network over Wi-Fi and start WMP as a UPnP server on that machine, do the SoundBridges and WMP on the Windows 7 machine see it? Does WMP on the laptop see WMP on the desktop?
Furthermore, please open "Network" on your Windows 7 machine and click on one of the SoundBridges. The SB's web interface will be opened in a browser window. Click "Status" and post the network information here (but without the MAC addresses, please).