Laura,

Since you said backup software I'm assuming you're using a third party solution for your SQL Server backups.  We've had problems in the past with backup software and it can sometimes cause problems/contention with SQL Server native backups by creating a blocking situation.

What I've done in the past has been to stop the backup software (in our case it runs as a service on the server), then stop and restart the SQL Server Agent.  Finally, remember to restart your backup agent service.  This solves the problem 99% of the time and doesn't require a restart of the server.

Hope this helps with your problem.