When you try accessing the computer by IP from the Start button, did you type \\192.168.1.5\Sharedfoldername?
The IP Address is just an example, yours will be different.

Do a test and Share your C or D drive and call it C or D for now and give it a full permission, then try accessing it again, for example: \\192.168.1.5\D

What's the name of the Security Software that you are using on both computers?