Using Microsoft's tool sqlmaint.exe you are able to create automated database backups for the express versions of SQL Server 2005 and 2008.

"C:\Program Files\Microsoft SQL Server\MSSQL10.SQL2008\MSSQL\Binn\sqlmaint.exe" -S .\sql2008 -D Northwind -BkUpDB "F:\backup" -BkUpMedia DISK -DelBkUps 4weeks

-S nameServer name
-D nameDatabase name
-BkUpDb PathAction to backup the database in the supplied path
-BkUpMedia MediaSpecifies the media type, either DISK or TAPE
-DelBkUps periodSpecifies the time, after which backups in the backup directory should be deleted.
number[minutes|hours|days|weeks|months]
e.g. 5days, 2weeks, 3months

Automated backups:

Create a batch file with your backup command (e.g. sqlmaint_full_backup.bat)

Use windows task planner to execute the batch file as required, daily, weekly, etc.

Further information:

http://msdn.microsoft.com/library/ms162827.aspx