Slow log on issues are mostly due to DNS problems.
Its difficult answer your question without specific details, would require some more details.
Which OS is running on clients XP, vista, Win 7 ?
how many DCs are there in your infra ?
how is your network setup, Single Office, Head Office - Branch Structure etc.. ?
Do you see any errors in event logs on DC as well as client machines ?
To start with, please refer similar threads...
Clients are very slow to login to the domain Windows 2003
http://social.technet.microsoft.com/Forums/en-US/winservergen/thread/2e340c9e-d71f-4b11-ae4f-4557b9188b09
User slow login into domain
http://forums.techarena.in/active-directory/1225437.htm
Slow to login when on domain.
http://social.technet.microsoft.com/Forums/en-US/windowsmultipointserver/thread/5d892880-8931-46d1-9fcb-4dc756ab3b05
Windows XP Slow login to Domain
http://social.technet.microsoft.com/Forums/en-US/windowsserver2008r2branchoffice/thread/f57ddd92-13ed-4422-b3c2-c849dc20e701
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