With the command 'id' you can see the user and group id which are needed in the mount command.
The file /etc/rc.local is executed at the end of the boot sequence, hence the vboxsf file system is defined. Add the mount command(s) in the rc.local file:
sudo vi /etc/rc.local
add the following line before the 'exit 0' statement:
sudo mount -t vboxsf -o uid=1000,gid=1000 xp_C /mnt/xp_C
where 1000 is the uid and gid of the default user in the ubuntu gest
xp_C is the name of the shared folder e.g. the disk C which you have to define first in the Devices/Shared Folders...
/mnt/xp_C is the directory you have to create first where the share folder is mounted