Was Windows running in "Safe Mode" when you installed the Guest Additions?
If not, reboot the Windows guest in "Safe Mode" and try it again (particularly if your guest is XP).
See Section 18.104.22.168, Installation [Guest Additions] in the User Manual; you will find that in /opt/VirtualBox/UserManual.pdf.
Too, before you install the guest additions, be sure that you've installed the VirtualBox Extension Pack (can't hurt, might help). You do that by starting VirtualBox (but don't start a virtual machine), click File, click Preferences, click Extensions then navigate to where your downloaded Extension Pack is and select it; it must be the same version as VirtualBox.
Hope this helps some.
Last edited by tronayne; 08-29-2012 at 02:33 PM.