SQLSERVR.EXE using more CPU,MSDB size is increased?

Hi
Environment
SQL Server 2005 enterprise Edition with sp2
Windows server 2003 with sp2

MSDB size grown up to 5gb why it was happened? where can i find the reasons? is there any SQL command to see what’s in MSDB?

another problem: suddenly SQLSERVR.EXE CPU Usage showing up 60%. what are the causes how to find those? and how to decrease CPU Usage. plz help me…