Well, if it comes on a VLAN (and this would be among the data your ISP gave you for connection) you need to “translate” it in the Mikrotik, assigning the WAN (usually ether1) port the appropriate vlan-id, like:
http://forum.mikrotik.com/t/fibre-isp-vlan-pppoe-configuration/174559/1
Depending on the specific ISP it may be needed to enable the DHCP client on the newly created virtual interface or set IP address/static route manually, like:
http://forum.mikrotik.com/t/vlan-isp-configuration/165250/1
or:
http://forum.mikrotik.com/t/isp-requires-vlan-300-tag/173025/1
Windows likely is giving the 651 error because you haven’t set the VLAN on it properly.