I installed Virtualbox and XP guest in Ubuntu 9.04,and enabled file sharing between them.But I encountered USB problem when I inserted a USB disk.Finally,I worked it out by following steps.
1.Open up VirtualBox,select the virtual machine and click on Settings.Then select USB in left and check the two checkboxes.
2.Open up terminal(Applications->Accessories->Terminal).Create usb group and add current user to the group:

sudo groupadd usbfs
sudo adduser your_username_here usbfs

3.Use this command,and you will get group id:

cat /etc/group | grep usbfs

Example of output:


Here,1001 is the group id.
4.Edit /etc/fstab by this command:

sudo gedit /etc/fstab

Add this line in the bottom.

none /proc/bus/usb usbfs devgid=1001,devmode=664 0 0

Here 1001 is the group id in step 3.
5.Run following command:

sudo chmod 777 /proc/bus/usb

Restart your computer.

I don’t konw whether it matters,but you’d better install guest additions (from Devices->Install guest additions in VM window)