How to make an NT User Authentication:
1. Create an local user group like "proxy_user" on the machine squid runs on. Everybody in this group will be able to use the proxy. 
2. Edit the squid.conf in \etc :
3. add: "auth_param basic program c:/Programme/squid/libexec/nt_auth.exe -a proxy_user" --this tells squid witch auth process to start and witch user group to use. Change the paths to fit your system!!! 
add: "auth_param basic children 5" --this is the number of auth programs that ar launced, each uses around 988k memory.
add: "auth_param basic realm What_you_want_to_say_to_the_user" --This ist the text witch appears on the auth window.
add: "auth_param basic credentialsttl 2 hours" ---this is the time the authentication is accepte by squid (if the browser is closed it wants a new auth) 
4. Create an acl : "acl mylan proxy_auth REQUIRED"
5. allow acces to the proxy: "http_access allow mylan" and at last secure it!!:  "http_access deny all"
6. Restart the squid service!
7. Test it
Bye!