Is the wifi password stored somewhere in windows-7?

In the computer that can connect to the network, type Manage wireless networks into the start menu search. Right click on the properties for the network you want and click Properties. In the security tab you should see an option to Show characters.

This will reveal your WiFi password that computer remembers.

This is the same process as described in the answer for Change the password Windows 7 has stored for a wireless network. Thanks @iglvzx for pointing this out.

You can retrieve a hex encoded version (which works if you input it into the usual places) with Nirsoft's Wireless Key View. The documentation for this states that the keys are stored either in the registry or the filesystem as follows

Windows XP: The wireless keys are stored in the Registry under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WZCSVC\Parameters\Interfaces[Interface Guid].

Windows Vista: The wireless keys are stored in the file system, under c:\ProgramData\Microsoft\Wlansvc\Profiles\Interfaces[Interface Guid]. The encrypted keys are stored in .xml file

The location for 7 and vista should be the same.

you can also export and store it in your own interface configuration file

netsh wlan export profile interface=wi-fi key=clear folder=c:\

it will generate an .xml file (named in a interface-essid format) in the folder specified (c:\) with the plaintext password (clear) inside of its keymaterial tag

then you can also easily import (netsh wlan add profile interface=filename=c:\interface-essid.xml) the configuration file where the essid and key are stored

it works on windows 8.1 as the interface is called wi-fi there (ncpa.cpl)

in windows 7 its name is wireless network interface

Alternatively you can export the profile of all profiles without knowing their name by using the wildcard * as interface name:

netsh wlan export profile interface=* key=clear folder=c:\

A different .xml file will be generated for each interface