MRTG SNMP settings on Windows

I'm trying to have xymon and MRTG show the network traffic graph for our
Windows servers.  I've got Xymon with MRTG functioning on an Ubuntu box and
it's able to make MRTG graphs for our router network traffic, but when I
try to monitor our Windows servers it shows no data in the graphs.  I've
got the SNMP service running in Windows and I've set the community name,
but no data.  I've opened up ports 161 & 162 on the firewall too.

Here's the error I get in the mrtg log on my xymon box.

2013-08-31 16:40:32: ERROR: Target[mrtg.vm-buzzard.lan][_IN_] '
$target->[3]{$mode} ' did not eval into defined data
2013-08-31 16:40:32: ERROR: Target[mrtg.vm-buzzard.lan][_OUT_] '
$target->[3]{$mode} ' did not eval into defined data

I'm sure it's something simple.  Please advise.

Thank you.
*Kris Springer*