VirtualBox log files is one of the great tools to diagnose an issue in your VirtualBox. It will always create a log file which reflects the lifecycle of your virtual machine.
VirtualBox log files is located in user/per-vm directory. Depending on the OS:
On Windows, %HOMEDRIVE%%HOMEPATH%\.VirtualBox\Machines\\Logs
E.g. C:\Documents and Settings\Username\.VirtualBox\Machines\\Logs\vbox. log
On Mac OS X, this is $HOME/Library/VirtualBox/Machines//Logs
On Unix-like systems (Linux, Solaris), $HOME/.VirtualBox/Machines//Logs
The log files are rotated such that the most recent is always called vbox.log and older ones are vbox.log.[123]. These log files contain lots of information about the capabilities of both the host and the guest vm and should be provided whenever reporting issues with VirtualBox.