I got the error message “unknown filesystem type vboxsf” while I was trying to share a directory from my Windows XP host to Ubuntu Interpid Ibex (8.10). In almost every where it said to install Guest additions. After spending few hours I found the solution. So decided to share with you for reference.
Step 1: Install host additions (usually you install them when you install virualbox).
Step 2: If you have created a new machine, then create a shared directory/folder. (From the machine’s Devices menu)