Do you have windows 7 installed? I don't know why this is happening, but I have had similar problems ever since I upgraded to Win 7. I never had this problem with XP on the same three systems. After I upgraded all three of my desktops to Win7 it started to happen randomly on all three systems at different intervals. I have a old Linksys Wireless G router and several computers connected to it. For no reason my adapters on different will suddenly disconnect. When I try to reconnect, it will show that there are no wireless networks in range, even if the computer is 3 feet from the router and other computers are connecting to it with no problem. The only way I have found to remedy this problem is to physically unplug the adapter and plug it back in. Disabling or uninstalling and reinstalling the driver while the adapter is still plug in does not remedy the problem, only physically unplugging it does. This of course is not possible with a laptop that has an internal wifi card. I guess maybe unplugging it and pulling the battery out might work.
Don't if it will work for you, but it might.