If this is Vista I had the same problem and in my case it was Norton and WIndows firewall. After uninstalling Norton, I had to use regedit and manually remove all instances of symantec(there were still a ton of dll’s loading). Then I had to go into the windows firewall through services and again re-disable it. For some reason Norton took control of the firewall but left pieces of the Windows firewall in place and in the background. From security center it thought it was disabled but under services it was not. I had to disable it there and suddenly it worked. If I remember Norton also by default turned off netbios which has to be enabled. I’ll try and remember what else I ran into. The newer images I’ve seen coming out seem to have a lot more of these issues fixed.