Seriously. I do not like Dell MediaDirect. It saws off four or five gigabytes of your hard drive for an entirely superfluous purpose. It also has a distressing tendency to stop working and lock your computer into a reboot loop; it will claim that it can’t read your hard drive due to BitLocker, reboot, and then start the vicious cycle all over again. And even if you repartition your drive and get rid of MediaDirect, if you accidentally press the MediaDirect button, the whole thing will start over again.
Fortunately, with Windows Vista, this is comparatively easy to fix.
Find a functional Windows Vista DVD and boot from it. Once you get to the menu, select Repair Your Computer, and from the listed repair options, select Startup Repair. Once Startup Repair has finished, let it reboot your computer.
Boot from the Vista DVD again (yes, you’ll need to run Startup Repair twice). Run Startup Repair again, and reboot. This time, boot from your hard disk, and Vista, not MediaDirect, should start properly this time.