@consoleadmin is probably on the right track.
I too am on Windows Server 2003 and I expect your hardware is probably pretty old like mine. I have exactly the same sort of problem because the sem5.log gets too large and the system bogs right down, thrashing the hard disk. This seems to cause the database service to not stop cleanly if required, and potentially it will then not start correctly due to a bad or too large sem5.log. Maybe you have a similar issue as me.
I have logged a support ticket with Symantec to disagnose why the log is not truncating as the current version should be doing this every 4 hours from what I understand. What version are you using?
Let us know how you get on with following TECH134782 and how big your sem5.log is.
Also verify the Database Engine process is in fact stopped before trying to start, its called dbsrv12.exe in the Windows process list if it still actually running.