Why is this so hard to understand?

I just want my app to check if the user's login to the app is OK or not. The login concists of a username & password. If they are known by the server I just want to ask the server if the combination is OK or not. That way I can change the passwords for my users on the server in the User Manager so their Login will change for Windows, their E-mail boxes, anything else and ALSO MY app. Want an example?

John has a username and password: Username is J123, password is asecret.
Now his login for windows needs the username & password, the mail-server requires the SAME password and my app will need the same password.

Now I (the administrator) go to the usermanager for the server and change his password to topsecret. If I wouldn't have done that his password could also have expired or been changed by himself (I know how to stop him from doing this and I know my share of NT...).
After the password change, he will have to use the NEW password for his next logon, and needs to enter the new password in his e-mail app again. Also my app will now know the new password, and will not accept the old one anymore. All I want to do is have my app ask the server if the password entered is correct yes or no. I don't need to know when the password is wrong what it SHOULD be. I just want to know if it's OK or NOT OK.
Why is it that other apps (like Winroute (see www.winroute.com)) can verify this, but I cannot?

What I'm trying to accomplish is a little module to import user-names and groups (I have accomplished this already) and to use a password verifying routine to check wether a password is correct or not.

I couldn't be more clear about this...please, if you don't get it now don't reply. If you understand the problem and have a suggestion or solution for it let me know...

Thanks.