This is not the case in Virtual box, sharing is accomplished using a special service on the host and a file system driver for the guest, both of which are provided by VirtualBox.To share a host folder with a virtual machine in Virtual Box, you must specify the path of that folder and choose for it a "share name" that the guest can use to access it.
After this step I'm expecting a new drive in "My Computer" as a typical Microsoft Virtual PC user. But there is no new drive. After digging into the help files, Virtual Box requires a explicit "net use" command to mount that shared drive.
net use x: \\vboxsvr\sharename
vboxsvr is a fixed name(might be helper service which runs on the guest along with Guest additions), "sharename" is the alias name which is provided while mapping the host drive
Virtual machine guest additions installation is a pre-requisite for folder sharing