It could be many different things. If you have been using any third party firewalls it is possible it may have screwed up access to your machine even if the firewall has now been completeley removed.
I also once had an error like this and after paying closer attention to what the full error said on the guest machine I realised a reinstall of the OS was the only way forward. This particular error was claiming that my server didnt have enough resources free for remote sharing, this was the same even after a cold boot with nothing loaded up in the foreground and nothing but windows own background services and only third party programs that where essential for network access.
I still to this day dont know what caused th error as my machine has plenty of resources for sharing, my old p200 with 32mb of ram could do it fine so why couldnt an athlon xp 1800 with 512mb ddr do it
