The reason for the errormessages is, that VFAT filesystem doesn't have any security bits in it like NTFS or Linux filesystems has.
To get rid of the error messages, you could add two parameters to the lines where you mount hdb1 and hdb5.
The parameters are users,quiet, so the hdb1 line would look like this:
/dev/hdb1 /mnt/Win vfat users,quiet,rw 0 0
The changes allows normal users to mount the partition, and when making changes on the partition, linux supresses the error messages. The rw part is unnecessary.