In the xampp\htdocs\xampp
folder you should see a .htaccess
file - delete that file.
You may also find a .htaccess
file in the xampp\security\htdocs
folder, delete that also.
Then try and go your admin page again.
If you can now access the admin page you may also be able to set a password again in the http://localhost/security
If Win7 is like Vista then you should disable UAC (User Account Control) and login in as an adminstrator to do most things in XAMPP.