Hi ,

To backup Active direvtory with backup exec you need to backup system state & root drive of that of domain controller or you with backup exec you can do GRT backup of active directory which help you in restoring single user in case of any corruption & helps in you in comparison too authorative restore

You can refer best practisces document for same too

http://www.symantec.com/docs/HOWTO21776--Active dirrecotry

You can also see best practisces document for other agents too

http://www.symantec.com/docs/HOWTO21789--Sharepoint

http://www.symantec.com/docs/HOWTO21793--SQl agent

http://www.symantec.com/docs/HOWTO21796--Exchange

Please got through this which will help you to configure your backup strategy

Please mark this post resolved if this helps you

Thank You