Hello,
You REALLY don't want to touch those files while SQL Server is running. SQL Server/MSDE has its own ways of backing those files up, which you have to use.
You can shut down the SQL Service and then back those files up
.
From what I remember, Backup Exec does have the ability (in the full version) to run scripts before and after your backups.
A simple before script that has the line net stop mssqlserver and an after script with the line net start mssqlserver will work wonders.
Thanks,
Mitch