Is the Virtual Machine's Network Adapter using Bridged or NAT (Share with my Mac)? Sounds like NAT however in either case you can add the NAS to the hosts file in the Windows 7 Virtual Machine and then you should be able to access it via its name.
If you have problems editing the hosts file, have a look at: You cannot modify the Hosts file or the Lmhosts file in Windows Vista and Windows 7
