The source database is probably damaged. Discard that backup, it's likely to be fairly useless. You shouldn't be worrying about the backup at this point, worry about the DB that the backup was taken of.
How often do you run CheckDB on the source database and when did it last run without errors?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability