Hello Darren,
Backupexec MSDE normally uses the SQL SA log in and password, unless you have chnaged it to something else before.
SA password blank by default.
Additional Information:
How to change the SQL Server (BKUPEXEC) system administrator password in Backup 9.x and 10.x for Windows Servers
http://support.veritas.com/docs/254374
please write back in case you need more Information.
Thanks
Regards