Hi Liya,
The error you get while backup of Exchange database is because of the internal corruption of the database which can be taken care of by a routine defrag of the database.
Also, during the database maintenance cycle you can pull the database offline and run the ISINTEG command (an exchange command for checking and fixing the integrity of the databse).
Instructions to follow the steps are mentioned.
http://support.microsoft.com/kb/301460
For any further queries you can contact me on my mail aatiquequreshi@walla.com
Regards,
Atique Qureshi
VCP, MCSE