Kevin Holman has written a post on how to  to discover a Windows Computer as a Network Device in SCOM 2012.

The design choice to not allow snmp discovers of windows devices was questioned heavily when it was announced and caused vendor management packs such as  Netbackup not to work.

This is a welcome discovery you can read the full article here. The below is for my record.

On the management server which will run the network discovery rules – browse to the following directory:
\Program Files\Microsoft System Center 2012 R2\Operations Manager\Server\NetworkMonitoring\rules\discovery

There is a file present in this directory named ic-post-processor.asl
Make a backup copy of this file, I like to name it ic-post-processor.bak, and place this backup copy in the same directly.  Now, edit this file, and change the following lines:

ISWINDOWSHOST(systemObj) do {
if (systemObj->Type == “HOST” && systemObj->Vendor == “MICROSOFT”) {
return TRUE ;
}
if (SEARCHSTRING(systemObj->Description, “Windows”)) {
return TRUE ;
}

systemOIDCheck = “.1.3.6.1.4.1.311.1.1.3.1” ;
if (substring(systemObj->SystemObjectID, 0, sizeof(systemOIDCheck)) == systemOIDCheck) {
return TRUE ;
}
systemOIDCheck = “.1.3.6.1.4.1.99.1.1.3.11” ;
if (substring(systemObj->SystemObjectID, 0, sizeof(systemOIDCheck)) == systemOIDCheck) {
return TRUE ;
}
return FALSE ;
}

to:
ISWINDOWSHOST(systemObj) do {
    if (systemObj->Type == “HOST” && systemObj->Vendor == “MICROSOFT”) {
        return FALSE ;
}
if (SEARCHSTRING(systemObj->Description, “Windows”)) {
return FALSE ;
}
systemOIDCheck = “.1.3.6.1.4.1.311.1.1.3.1” ;
if (substring(systemObj->SystemObjectID, 0, sizeof(systemOIDCheck)) == systemOIDCheck) {
return FALSE ;
}
systemOIDCheck = “.1.3.6.1.4.1.99.1.1.3.11” ;
if (substring(systemObj->SystemObjectID, 0, sizeof(systemOIDCheck)) == systemOIDCheck) {
return FALSE ;
}
return FALSE ;
}