How is the Host connected to the LAN, Ethernet or Wi-Fi?
What type of Router in used on the LAN?
How is the Guest's Network Adapter configured, Bridged, NAT, Host-Only or LAN Segment?
In Windows 7 what type of Network are you selecting, Home, Work or Public?
In Windows 7 is the Network Adapter configured for DHCP or have you set a Static IP Address?
If DHCP, have you tried setting a Static IP Address?