When I add a new username, the client signs on for the first time in his computer, and everything is fine. When the client signs on for the second time, the network rejects the username and password. When you type it, it gives the message that username and/or password are invalid. This has happened with the last three username accounts that I have created. Who can figure this one out?

What kind of network is this....Peer-to-Peer or a Domain? Are the clients trying to log onto clients PC's or locally on the Server?

Domain. They're trying to log on to the domain.

your problem is that when you create the account your foregetting to un-click the "user must change password at next logon" box. try clicking the "password never expires" box instead.

I have it on that setting "Password never expires"

hmmm, interesting, is there anything at all that these users are doing when they log on the first time?? or will the error occur if they just log on, log off and try to log on again?

The error occurs when the user logs on again the next morning, when they boot up their computer. The first time after I created the username, they log on fine. This one is a strange one.