Your router would be suspect here, have you updated its firmware as a possible solution? And I
would update your WiFi drivers on the computers. How far are you positioned from the router?
Are there obstructions in the way?
Try actually updating your driver and clearing the network logon.
Control Panel - Network - write down make and model of Wifi - double click on it - driver's tab - write
down version - click Update Driver (may not do anything as MS is far behind certifying drivers). Then
Right Click on Wifi Device and UNINSTALL - Reboot - this will refresh the driver stack.
Download - SAVE - go to where you put it - Right Click - RUN AS ADMIN.
You can download several at once however reboot after you install each one.
After looking at the System Maker's you can check Device Maker's for even newer versions.
(The ones from System maker become your fallbacks).
Repeat above for network card (NIC) and is a good time to get other updated drivers as Windows
7 loves updated drivers.
I would also disable auto-updates for drivers. If Windows Updates suggests one just HIDE it as
they are almost alway old and you can look for drivers manually as needed.
Make sure you know the login details for your wireless router - SSID and Password.
You will lose connection when doing this and have to redo your logon.
Control Panel - Network & Sharing Center - on Right Side click Customize - Set Network
Locations page - on lower left click Merge or delete networks locations - DELETE ALL
instances of your network (and any others you no longer use) - REBOOT. Start - Connect
To to logon on to Network.