Problem With Login Null

Hi,
I have a problem. My web server is a Windows 2k Server and my database server a Windows 2003. I use windows authentication to login at SQL Server, but when I access from a client machine that uses Win 2k Professional I always receive this message: "[Microsoft][ODBC SQL Server Driver][SQL Server]Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection.".
When I access from a client machine that uses Windows XP everything it´s ok.

This problem only occurs at the web site. When I try to access the Query Analyzer or the Enterprise Manager I still have the connection.

Can anyone help me?!

Thanks,

Riane Santos
riane@cpunet.com.br