i posted this in another but similar topic and hope this will help you. I had the same problem before. If you are using Windows 7 and installed the wifi driver intended for Vista, you have to uninstall your wifi device from the device manager and delete the driver with it. Let your computer boot up again and let Windows 7 install the generic wifi driver. After i did that, my wifi disconnection problem was gone.
I was only able to locate the exact cause because i noticed that when i connect my laptop to the router wired i dont have the disconnection problem. The disconnection only happens when im connected wireless.