Anyone heard that it's possible to login to Windows xp programmatically, I want to modify the way the Domain Controller authenticates the user and if the user authenticated in my special way it returns the user name and password to the service working on the client machine so it should login the user using its passed credentials from the DC without waiting for user to insert his original credentials.
Have a look at this: Gina.dll MSDN
Gina is the system that fingerprint readers etc. use to customise the login screen. You may be able to use this to achieve your purpose?
To future proof your app (Vista and Windows 7) you may wish to look into these: MSDN Mag Vista Credential Providers