Try This:
At First Download The Windows Server 2003 Resource Kit Tools
Windows Server 2003 Resource Kit Tools
http://go.microsoft.com/fwlink/?LinkId=4544 after installation is complete, click on: Start, All Programs, Windows Resource Kit Tools, Command Shell
Then enter the following commands. (Attention: they are case sensitive.)
net user guest /active:yes
ntrights +r SeNetworkLogonRight -u Guest
ntrights -r SeDenyNetworkLogonRight -u Guest
The first command enables network access for Guest, the two subsequent ones change two different policies to allow network access for Guest.