You might check an see what program start when you restart your system... many 3rd party programs which are used as CLEANERS, auto remove ALL PASSWORDs, That would be the first thing I would check.
If it is the Computer, simply login to any forum.... make sure your logged in... and have Win 7 set to REMEMBER PASSWORDS.
then close that forum, move on to others forums or things, then come back to this forum... if you are auto logged in. it is not WIN7.
Many forums offer 10 or 20 things to do which make your System FASTER.... well, make sure you read and understand everything they recommend you do ...one of these faster things is disable: AutoComplete may be disabled, you can check by going to Tools, then Internet Options. On the Internet Options screen go to the Content tab, and click the Settings button in the AutoComplete section. In the Settings make sure that User names and passwords on forms is checked.