Joe, I may have seen this before. Similar to Nelo's recommendations, perform the following checks:
Settings > All Settings, expand Settingsn > Deployment and Migration > Symantec Boot Services (PXE) and select PXE Server Configuration. Is the site server listed?
If the site server isn't listed, start all four SBS services on the site server. Do they remain started, or do some crash? If they crash, a possible cause can be an IP address change or missing IP address in initialPXEConfigPath.txt. Open this file located at <drive>:\Program Files\Altiris\Altiris Agent\Agents\Deployment\Task Handler\SBS\initialPXEConfigPath.txt and ensure the following two lines are present, with correct IP address:
Server_Listeners="10.3.0.1" (or whatever your site server's IP is)
Mtftp_Listeners="10.3.0.1" (site server's IP)
If this information was missing, or even if it was correct, reset this registry key to 0:
HKEY_LOCAL_MACHINE\SOFTWARE\Altiris\Deployment\Symantec Boot Services\HasSbsRegistered
At this point, if you restart the services, you should see it appear in the list on the PXE Server Configuration page. If not, and if no PxeStartupInfo.log file is present, try starting the process that populates this information by manually double-clicking PxeStartupInfo.exe in the SBS folder mentioned above.
Does this resolve the issue?