The hard disks of your virtual machines are located as described below (by default):
- For Windows Vista and 7: C:\Users\username\.VirtualBox\HardDisks
- For Windows XP: C:\Documents and Settings\Username\.VirtualBox\HardDisks
- For GNU/Linux: ~$/.VirtualBox/HardDisks
- From there you can copy the files that are in the directory on an external media (backup your virtual drives).