backup system drive?

I'm getting conflicting info.  one book says to backup everything C: (system) and D: (exchange drive) completely along with system state info at least once a week.  Another source says that it is not important to backup the C: drive because it is better to rebuild a server from scratch at that point. 

 

I currently only backup my data drives, system state, information stores, mailboxes.

 

Do you bakup your C: and D: drives completely?

how is backupexec able to backup the C: drive when so many files are in use (services, etc...)?

how would I even go about restoring the C: drive if my system crashed and I was unble to use backupexec?

 

Thanks