I mount my shared folder in /etc/fstab like:
sharedsfname /files vboxsf rw,noexec,auto,nouser,uid=1000,gid=1000 0 0
The uid/gid=userid option is the key in my case... Without it I have the same issue that you are experiencing.
I have a linux on linux setup though, so I am not sure about the effects with a Windows host.
In my example
I am creating my shared folder and checking 'make permanent' (always clicking OK, to accept the changes)
I am creating my own folder on my guest called /files.... owned and r/w by my user.
Using the shared folder name (NOT THE AUTO-NAME mount point, created by virtualbox?: sf_XXXXXXXX), This is the name that you designated in the virtualbox GUI on the host(it also gets auto-created but it is an editable field in the Shared Folders editor)... you can also get this name from sudo VBoxControl sharedfolder list
on the guest. Mine is called sharedsfname.
Adding a line (above) to /etc/fstab, designating the share sharedsfsname, the mount point /files, and the filesystem type vboxsf, along with some options... most importantly the uid=1000 and the gid=1000. Those are the id of my user.
After that, you should be able to mount the drive by typing sudo mount -a
. Or by rebooting... The auto option ensures mounting on startup.
Virtualbox wanted you to be able to effectively do the same thing in its GUI, and for the most part you can. In the Shared Folders editor I can designate the (host's) share name and I can also give a path for the mount point of directory you wish to use as a mount point on the guest, like /home/wu-tang/data... it will mount your share if the /home/wu-tang/data exists.... or it will create /home/wu-tang/data and mount your share... On my system it gives the mount point ownership of root and the group of vboxsf (root:vboxsf). In this case you will need to add your user to the vboxsf group and have access. (Which is why I prefer the previous mentioned method instead because you are simply giving ownership to your user without any additional alterations). But as soon as you exit the editor, the mount seems to happen on the guest. I could not see any options using VBoxmanage to create the share on the command line with specific permissions/ownership to avoid the extra permissions related steps.
minor frivolous rant here...
Another thing I just discovered that I don't like, is that it will create these folders if they don't exist, REGARDLESS of where you tell them. Be careful, the user is running the virtualbox manager and is able to create directories on the root partition of the guest. I created a second varlog directory on the guest / and it did not allow the virtualbox host to designate it as a mount point, so there 'seems' to be some safety built in there. Though the host's user would technically be trashing the user's VM, if things went wrong, so logically, it is not that alarming.