If this shows up when trying to access files on another computer chances are that computer does not have Guest access enabled. Enabling Guest access is not recommended, but sometimes when dealing with two systems with different operating systems this can help you get past a permissions error like the one above.  Make sure you have a good firewall and antivirus/antispyware suite installed on both machines before attempting. That being said, here is what you can do on the computer containing the shared files:
Download this program and install it:
Windows Server 2003 Resource Kit Tools
http://go.microsoft.com/fwlink/?LinkId=4544
Once installed, click on  Start–> All Programs–> Windows Resource Kit Tools–>Command Shell
Then enter the following case-sensitive commands.
net user guest /active:yes
ntrights +r SeNetworkLogonRight -u Guest
ntrights -r SeDenyNetworkLogonRight -u Guest
The first command enables network access for Guest, the next two change two different policies to allow network access for Guest.
