You can tell it's likely to be a Windows box by what ports it has open...
"The target host OS can often be guessed simply by looking at the ports which are open. Microsoft Windows machines often have TCP ports 135 and 139 open. Windows 2000 and newer also listen on port 445." ( from: http://nmap.org/book/osdetect-other-methods.html )
Once you've identified it's a Windows host, how can you tell which one?
Try this:
$ nmblookup -A 12.34.56.78
Looking up status of 12.34.56.78
HOSTNAME <00> - M <ACTIVE>
DOMAINNAME <00> - <GROUP> M <ACTIVE>
HOSTNAME <1f> - M <ACTIVE>
HOSTNAME <20> - M <ACTIVE>
DOMAINNAME <1e> - <GROUP> M <ACTIVE>
MAC Address = 00-11-22-33-44-55