Whilst trying to backup a SQL Server 2005 database on Windows 2003 I was given an error which said, amongst other things operating system error 5. That didn’t help a great deal, but the resolution was relatively simple: give the SQL user on the server access permissions to the folder I was trying to back up to. On my server the user was along the lines of SQLServer2005MSSQLUser$…..
More information can be found on Microsoft’s knowledge base.