Had similar problem when our system administrator renamed the production db server (win 2003) without telling me (the DBA).
Check out BOL:
Although SqlServer (2000) continues running quite happily - SqlServer Agent got very confused - it carried on running all the scheduled backups etc. but I could not edit any of them (or stop, start, re-schedule them) when logged in as BUILTIN/Adinistrator.
Changing the server name at NT admin level does not update the server names listed in the MSBD system database. Which explains why SqlServer Agent wasn't aware of the new server name.
I hacked around in the MSDB system tables (which exists solely for the benefit of SqlServer Agent) and fixed the problem manually by changing column data (server_name) in one of the columns.
THIS IS NOT RECOMENDED !!! but, I got away with it - and learned a lot more about SqlServer.
THIS IS THE RECOMMENDED METHOD....
I later found this in BOL (Sql 2000):
When you change the name of the computer that is running Microsoft® SQL Server™ 2000, the new name is recognized during SQL Server startup. You do not have to run Setup again to reset the computer name.
You can connect to SQL Server using the new computer name after you have restarted the server. However, to correct the sysservers system table, you should manually run these procedures: