Windows 2003 Server SP2
Initial problem was no desktop, icons or taskbar for a logon at the console and mstsc logons would get an error.
This was traced to a bad key in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\explorer.exe
"Debugger"="C:\\Program Files\\Microsoft Common\\svchost.exe
Once that key was removed, the machine rebooted with no problems.
The machine runs fine and I can mstsc to it fine. After some time, the key returns and I cannot mstsc to the machine. If I reboot, no desktop. so I remove the key reboot and all is well.
Symantec Antivirus did not find anything.