Error when starting SQLSERVER AGENT

Hello!, I have the following error message when I want to start the
sqlserver agent: "The SQLSERVER AGENT SERVICE on local computer started and
then stopped. Some services stop automatically if they have no work to do,
for example the performance logs and alerts service"
In this server I have windows 2003 and sql server 2000 (it has the service
pack already), actually some days ago it had no problem, suddenly the jobs
did not run so I realized this problem.
Help!!!!!!!!
Message posted via http://www.droptable.com
The MSDB database has some problem,maybe it is running in single user mode.
maybe you moved it from another BD check the MSDB for any thing out of the
normal.