Create an Identical UserName and Password (same as your LogIn) to Cesar's computer.
Make sure than Netbios is enabled. Here's How
If his computer has Windows firewall enabled, make sure that Files and Printer sharing is excluded.
Any Security software can also be a culprit if there's one installed. Disable it for now.
From your computer try accessing his computer using the IP Address instead of a computer name, and see if that will make a difference.
Click on Start and type \\IPAddress\C
(cesar's computer's IP Address and C is the shared drive)
Please post an update.