There are some bugs in the VirtualBox "Shared Folders" feature.  It can be convenient to work-around this in a secure way by using the Microsoft Loopback Adapter.   Here's how I set it up for a Windows XP guest running on an XP host.

• Set up the Microsoft Loopback Adapter on the host:
• Open Control Panel, and start "Add Hardware"
• Select "Yes, I have already connected the hardware"
• Select "Add a new hardware device"
• Select "Install the hardware that I manually select from a list"
• Select "Network Adapters"
• Select "Microsoft" and "Microsoft Loopback Adapter"
• Assign the static address 192.168.2.1 to it.
• Next?