You need to ensure that the username on each system is the same, and that the password is identical to get it to work flawlessly. Alternatively, you can try to connect to the other PC by going to Start, clicking Run (or press Windows + R on the keyboard) and typing \\yourpcnamehere and hitting enter. It SHOULD prompt you for a username/password if you've set up folder shares already. Then just enter the username & password on the other PC to get in.