I recently needed to recover a long forgotten WPA encryption key from a friends Windows XP laptop.  Unfortunately the wireless router password and the ISP credentials were MIA also, so changing the key or resetting the router were not options.  After some searching I found WirelessKeyView from Nirsoft (http://nirsoft.net/utils/wireless_key.html).  This is a simple EXE download that displays the WEP or WPA key for all networks on the laptop.  For Windows XP it can only get the 64 digit hex key because XP doesn't save it in clear text.  However this will work fine when joining the network.  On Windows Vista and Windows 7 it will retrieve the key in ASCII.