To find your password for a WiFi network you've already connected to in Windows 7 do the following:
Click on the Network icon (shown as seen when connected to WiFi):
Select "Open Network and Sharing Center"
Select "Manage wireless networks" in the left hand pane
Double click the desired WiFi network
Select the "Security" tab
Tick the "Show characters" check box
Make a note of the password
Click "Cancel" to prevent any changes being saved and close any open windows from the previous steps