Yes, but... The Win XP Pro installation on my computer is 7.8 GB, so an external
hard drive is a more workable choice. The basic Win install is found in
/home/<yourusername/.VirtualBox/HardDisks/

Then there are the snapshots in /home/<yourusername/.VirtualBox/Machines/

I recommend removing all but the latest snapshot before backing up. Do not
simply delete them from /home/<yourusername/.VirtualBox/Machines/, but remove
them through the VirtualBox control panel. A proper backup of your Win install
should use about 20 GB.