Windows 2003 network problem

Clients on my Windows 2003 Server controlled domain are suddenly blocked or very slow in getting access to the server. Explorer times out. Logins and terminal server are very slow (5-10 minutes) if they work. SQL is usually ok, but I think that traffic goes through a different channel. Mapped drives don't work. However, traffic from the server to the clients is fine (e.g. I can copy files to a client without any problem). I need someone to figure out what has gone wrong (configuration, virus, etc.), to explain what happened and to fix it. There is remote access through Hamachi/VPN which I think may be faster, and I can be at the computer or on another computer beside the server if it would help solve the [login to view URL] network has about 15 clients and 1 Dell 1800 server. The change appears to have happened very suddenly.