Database Backup Error

I have an ASPNET webform to manage sql express 2005 backups.
If i test the application with the debugger and click the button to made a backup, everything works ok and the backup is created inside my web application folder. When i run the web app under IIS, if I try to do the backup i recieve the following error:

Server Error in '/test' Application.
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User does not have permission to perform this action.
User does not have permission to perform this action.
Processed 416 pages for database 'test', file 'test' on file 5.
Processed 3 pages for database 'test', file 'test_log' on file 5.
BACKUP DATABASE successfully processed 419 pages in 0.754 seconds (4.543 MB/sec).

I have included the <localmachine>ASPNET user in my database with
schema 'dbo' and 'db_owner' permission. I have also change the security directives of the folder where the backup is going to be saved (write permission,creation,etc..) but it doesn´t work.


It seems as if the ASPNET user have not enough permissions to do the backup operation
Any Idea??