Shared folder between MacOSX and Windows on Virtual Box

You should map your virtual network drive in Windows.

1. Open command prompt in Windows (VirtualBox)
2. Execute: net use x: \\vboxsvr\<your_shared_folder_name>
3. You should see new drive X: in My Computer

In your case execute net use x: \\vboxsvr\win7

Edit

4+ years later after the original reply in 2015, virtualbox.org now offers an official user manual in both html and pdf formats, which effectively deprecates the previous version of this answer:

• Step 3 (Guest Additions) mentioned in this response as well as several others, is discussed in great detail in manual sections 4.1 and 4.2
• Step 1 (Shared Folders Setting in VirtualBox Manager) is discussed in section 4.3

Original Answer

Because there isn't an official answer yet and I literally just did this for my OS X/WinXP install, here's what I did:

1. VirtualBox Manager: Open the Shared Folders setting and click the '+' icon to add a new folder. Then, populate the Folder Path (or use the drop-down to navigate) with the folder you want shared and make sure "Auto-Mount" and "Make Permanent" are checked.
2. Boot Windows
3. Once Windows is running, goto the Devices menu (at the top of the VirtualBox Manager window) and select "Insert Guest Additions CD Image...". Cycle through the prompts and once you finish installing, let it reboot.
4. After Windows reboots, your new drive should show up as a Network Drive in Windows Explorer.

Hope that helps.