Try to start SQL Server Agent from the command prompt using:

**SQL 2005

SQLAGENT90 -C -V>C:\SQLAGENT.OUT

***SQL 2008

SQLAGENT.exe -C -V>C:\SQLAGENT.OUT

If this is u r error:

SQLAGENT.OUT:

Unable to rename C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\LOG\SQLAGENT.OUT to...