cannot write file

i'm trying to write to a file using asp.net. I've set all security settings correct (I think), for aspnet and ius_<machinename> i set read and write access. i disabled read only on the folder.
However, I get the following error:
 
System.UnauthorizedAccessException was unhandled by user code
  Message="Toegang tot het pad C:\mp3\data\Zip is geweigerd."
  Source="mscorlib"
  StackTrace:
       bij System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
       bij System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy)
       bij System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare sha ...