Windows 7: Windows cannot access:\\Pc\Name

Hi,
I hope someone can help me with some homegroup and networking problems.

1) My main computer is a desktop PC, which is in my upstairs study. It is connnected wirelessly to the internet, and the Virgin router is downstairs in the living room. This has worked perfectly for 2 years. The computer uses Windows 7 ultimate. (COMPUTER 1)

2) I recently purchased a laptop (COMPUTER 2) and installed Windows 7 ultimate. This uses the same wireless connection and at first I had no problem setting up a homegroup so that I could watch videos stored on Computer 1 on the laptop. The first problem is that the homegroup stopped working some time ago and when both computers are switched on, I get the message on both computers: "no other homegroup computers are currently available." I figured I could leave the homegroup and start again but if I try to leave on either computer I get the error message: "Windows couldn't remove your computer from the homegroup." In services, both the Homegroup settings (listener and provider) are started, as are the peer networking options. I have tried disabling them and re-starting to no avail. Now, this hasn't been a major problem because I have still been able to access files on Computer 1 via Computer 2 by clicking on network>Computer1>user. I can cope with this arrangement if that is the only solution.

3) But here is where it gets complicated. I have now purchased another desktop PC (Computer 3) which I intend to place in the living room and connect to my TV, so that I'm not constantly connecting wires from my laptop (computer 2) if I want to watch something on the big screen. Again, Windows 7 ultimate installed. However, I don't have a USB wireless adaptor for this PC so I am using an ethernet cable. Is this a problem? As I feared, it won't let me join the homegroup for whatever reason. More importantly, although I can see Computer 1 in the network option (and computer 2 when switched on), I get the message: "Windows cannot access:\\computer1\user. As far as I can tell, all settings on computer 1 enable sharing without passwords and allow "everyone" to access the files.

I hope that makes sense and thank you for any help in advance.

Alan.