PC Connection Times Out when try to add share

I am using black Apple Tv software 4.3 with recently installed firecore.

I want to stream video from a Windows 7 PC.

I have three Windows 7 PC’s on a wired network. All have file sharing switched on with permissions to Everyone and no requirement for passwords or user names. The three PC’s can access each other’s files no problem. They are in a WORKGROUP but not in a HOMEGROUP. Windows firwall is switched off, but they all use an ESET firewall. As far as I can tell, all file sharing set ups are identical on all 3 PC’s.

When I try to connect the Apple TV using “Add Share”, it identifies all 3 PC’s. 2 of the PC’s connect fine. However, the third just times out when I try to test the connection. Of course, it is the third that houses all my media and is in the same room as the Apple TV. The other 2 PC’s are my sons’ gaming PC’s, up 2 flights of stairs and switched off much of the time, so it would not be a very practical solution to transfer the media files to one of them.

Does anybody have any idea what I might be missing to enable a connection to this third PC? The only difference I can think of is that Windows has 4 user accounts, whereas on the other two machines they only have one user account each. I have tried using one of the user names on this main PC in the “Add Share” settings but this makes no difference. No passwords are needed for any of the user accounts.

All PC’s have DCHP ip address settings. I could give the third PC a static address, although I have a memory when I tried this before for another reason it caused difficulties in my internet connection (Virgin cable).

I have run out of ideas, so any help would be welcome.