I have been banging my head against a wall with this issue for some time but I have finally found the solution.
If you are having trouble with Windows 7 not saving your network security key, or reverting to some old key, the solution is as follows:
run msconfig (start > run (or just type into the "Search programs and files" field above the windows orb) > msconfig)
this brings up the System Configuration dialog box
click the Startup tab
scroll to find your wifi adapter's startup program (in my case it was NETGEAR)
uncheck the box next to this startup item
restart your computer. problem solved.