Using AppleJack, you can repair your disk, repair permissions, validate the system's preference files, and get rid of possibly corrupted cache files. In most cases, these operations can help get your machine back on track.
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers, it is also the only professional solution that is freely available as Open Source Software under the terms of the GNU General Public License (GPL). See "About VirtualBox" for an introduction.