On a server with more than one network card, the PXE services - specifically the SBS Server and MTFTP services - don't bind to the correct network device, thus not receiving incoming packets from the clients to allow them to PXE boot.
Often this will simply result in a "PXE-M0F" message on the client machine, after which it will continue to boot to the Production Operating System.
PXE services are only able to bind to and listen on one IP address, and will bind to the first NIC made available to it by the Operating System.
Change the Binding Order:
In the "Network Connections", under "Advanced Settings", the binding order can be modified. Select the desired device and move it to the top for PXE services to bind to that device at start up.
Change the Value in the Database:
In the database, there is a table called "sbs_ServerList". For the record with the corresponding Guid (matching the server), the "IP Address" column needs to be changed to the IP address of the NIC to which the PXE services will be binding.
Modify the initialPXEConfigPath.txt:
In the folder "<Program Files>\Altiris\Altiris Agent\Agents\Deployment\Task Handler\SBS" the file "initialPXEConfigPath.txt" needs to be modified. In the file, the fields "Server_Listeners" and "Mtftp_Listeners" need to be set to the new IP address of the server.
**NOTE: Sometimes the server will need to be rebooted for these changes to take effect and the PXE services to bind on the newly designated NIC.