I would also like to add that its best to give the 2 computers static IP addresses as for I have seen issues with Windows getting messed up when dynamic IP address changes etc, especially with mapped drives between systems.
Once I changed from system name\share mappings to static_IP\share mappings such as below:
\\system_name\share$
to
\\Static_IP\share$
The problems I had went away after setting up Static IP's and Static Mapped Drives. I was really scratching my head as to why this was a problem when they were both on the same version of Windows 7 Home Premium, same workgroup, router, and the security settings were configured so that the firewall allowed for both systems to access each other but no others to connect to them.
I was having same issue where it would work for a while and then a few days later wouldnt work. Going Static Fixed This!!!!
I have since changed my setup to not conflict for bandwidth with other devices in my home using the network. I installed second NICs in these 2 systems and put a cross over cable between them, since after all, i am using Static IP addresses. So now I can transfer large files between 2 systems such as a 18GB video game from one system to another without affecting everyone else that is watching netflix etc.