Assuming you have not installed the 'Virtualbox Guest Additions' (you do not mention if you have done so in your question) you simply need to install them reboot the virtual machine and then sharing folders should work.
In the "Devices" menu in the virtual machine's menu bar, VirtualBox has a handy menu item named "Insert Guest Additions CD image", which mounts the Guest Additions ISO file inside your virtual machine. A Windows guest should then automatically start the Guest Additions installer, which installs the Guest Additions into your Windows guest. Other guest operating systems (or if automatic start of software on CD is disabled) need manual start of the installer.
For the basic Direct3D acceleration to work in a Windows Guest, you have to install the Guest Additions in "Safe Mode". This does not apply to the experimental WDDM Direct3D video driver available for Vista and Windows 7 guests, see Chapter 14, Known limitations for details.
If you prefer to mount the additions manually, you can perform the following steps:
Start the virtual machine in which you have installed Windows.
Select "Mount CD/DVD-ROM" from the "Devices" menu in the virtual machine's menu bar and then "CD/DVD-ROM image". This brings up the Virtual Media Manager described in Section 5.3, “The Virtual Media Manager”.
In the Virtual Media Manager, press the "Add" button and browse your host file system for the VBoxGuestAdditions.iso file:
On a Windows host, you can find this file in the VirtualBox installation directory (usually under C:\Program files\Oracle\VirtualBox ).
On Mac OS X hosts, you can find this file in the application bundle of VirtualBox. (Right click on the VirtualBox icon in Finder and choose Show Package Contents. There it is located in the Contents/MacOS folder.)
On a Linux host, you can find this file in the additions folder under where you installed VirtualBox (normally /opt/VirtualBox/).
On Solaris hosts, you can find this file in the additions folder under where you installed VirtualBox (normally /opt/VirtualBox).
Back in the Virtual Media Manager, select that ISO file and press the "Select" button. This will mount the ISO file and present it to your Windows guest as a CD-ROM.
Unless you have the Autostart feature disabled in your Windows guest, Windows will now autostart the VirtualBox Guest Additions installation program from the Additions ISO. If the Autostart feature has been turned off, choose VBoxWindowsAdditions.exe from the CD/DVD drive inside the guest to start the installer.
The installer will add several device drivers to the Windows driver database and then invoke the hardware detection wizard.
Depending on your configuration, it might display warnings that the drivers are not digitally signed. You must confirm these in order to continue the installation and properly install the Additions.
After installation, reboot your guest operating system to activate the Additions.