I was getting really frustrated with this problem when I was trying to connect a Windows 7 PC to my Microsoft Hyper-V 2008 R2 server. No matter how many posts I read, nothing would work. Until one post made me think to double check my IP address settings on my Hyper-V box.
Low and behold my subnet mask was incorrect. Once that was changed to the correct setting, all was fine in the virtual world.