Windows 7: HG Sharing not working for no apparent reason

I had a homegroup on my network, composed of one desktop computer and 4 laptops (I'll call them Desktop, H, D, M and K for clarity). For a few weeks, everything was working fine.

However, today I was on K and it was for some reason unable to connect to any of the other computers. I ran the HG troubleshooter, which didn't help whatsoever, then left and rejoined the homegroup. Now K and Desktop could both see each other, and Desktop could access all of K's shared folders, but K could only see Desktop's libraries and not access them. The library icons looked like this (link) and when I click on them nothing happens. K had full access to all the folders on H, but couldn't see M or G at all.

Frustrated, I had every computer leave the homegroup and started a new one on Desktop. Or, I tried. On M, it said "Windows was unable to remove you from the homegroup" with no further explanation as to why.

So now, Desktop, K, G and H are all on the same, brand new homegroup.
Desktop and G have full access to all the other computers and their files, no problem.
K has full access to H, but has the same problem as before when trying to access Desktop, with the folders visible but somehow inaccessible. K can't see G at all.
H has full access to G, and can see Desktop and K and their folders, but can't open any of them.
M is still stuck in the old homegroup, and can't leave for some reason.

I've run the HG troubleshooter on all of them, and had them all leave and rejoin the current homegroup at least once.

WHAT is going on?

You might want to try going through this Homegroup trouble shooting list to see if you have everything set up correctly.

It's also important to realize that you need to share some of your Homegroup folders in order for your machine to appear in the Homegroup of other machines, the picture below shows the correct permissions for this.

Ultimate Troubleshooting Guide for Windows 7 HomeGroup Connection Issues

For the can't leave the Homegroup problem you need to perform the following procedure on the machine with the problem.

The fix seems to be Deleting the file idstore.sst under C:\windows\serviceProfiles\LocalService\AppData\Roaming\PeerNetworking


You may need to go back into services and enable Peernetworking again after the fix. Don't forget to restart all machines after the fix.

You should be able to leave the homegroup after that.

As for the other problems with shares disappearing, which A/V software do you have installed?

Everything on that troubleshooting checklist is set up properly.
The sharing permissions are all correct, I checked those as well earlier.
I managed to get M off the old homegroup and onto the new one, and everything is working fabulous on that one now, it can access all the computers and folders, so thanks. :3
As for AV, the desktop has none, two laptops have AVG 2011, 2 have avg 2010 and one has none.