VirtualBox Windows XP guest boot in safe mode

In this case, F8 won't work. fn+F8 won't work too since here it's a special function such as open itune. The method is:

With the XP guest running.
Go to Start, click on Run. In the box type msconfig and hit enter.
Click on the BOOT.INI tab, Check /SAFEBOOT save and restart.

Once GA are installed, reverse the above and reboot.

See here.