1) Connected?
2) Can you log in using a cached account or bypassing (win9x) and ping the server?
3) Other clients having the same error?
4) Windows NT/2000 system? Try removing and replacing the client through Server Admin on the DC.
5) Synchronize your domain.
6) Network seriously busy? or going over a Wan link?
7) any other information you think might be pertinant.