Windows has some restrictions about files in the root of a drive, especially C drive. Rather create a folder and write the backup into that. Make sure that the SQL Server service has full control over whatever folder you use. Operating system error 5 is Access Denied.
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