if your msdb database has been marked suspect and you are unable to take back up from maintenance plan then follow this procedure

This is for sql 2000

1.. In SQL Server Enterprise Manager, right-click the server name and

click Properties.
2.. On the General tab, click Startup Parameters.
3.. Add a new parameter as "-T3608" (without the quotation marks).
After you add trace flag 3608, follow these steps:
1.. Stop, and then restart SQL Server.
2.. Make sure that the SQL Server Agent service is not currently running.
3.. Detach the msdb database as follows: or use the query msdb detach query.
use master
go
sp_detach_db 'msdb'
go
4.. Delete or rename the msdb mdf and ldf files( msdb.log , msdbdata )
5.. Run the INSTMSDB.SQL script ( use search option to find out the script file. This file will be available in your
server itself)

6.. Remove the -T3608 trace flag from the startup parameters box in
Enterprise Manager
7.. Stop, and then restart SQL Server.