Check the following:
1 is workgroup name the same on the each machine, if it isn't make it so

2. Is the machine name differrent for each machine, if not make it so.

3. Is each machine uniquely named, that means no machine has the same name as a user, if not make it so.

4. Is there a common user/password on each machine, not the Administrator, if not make it so.

5. Does each machine have an ip number (192.168.1.x) and is each number unique x, x+1 x+2 etc.

6. The ipconfig command will answer that question.