Try this: Restore Point:
Try tapping F8 at startup, and from the list of startup selections, select Safe Mode by using UP Arrow Key to go there > then hit Enter.
Try a System Restore once there, to pick a Restore Point before your problem..
Click Start > Programs > Accessories > System Tools > System Restore > pick a different time > Next > etc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If Restore does not work and you do not have a Microsoft Vista DVD, make a Bootable Repair Disk to do a Startup Repair:
Download the ISO file at the supplied link, and make a Bootable Startup Repair Disk from it.
Go into your Bios/Setup, or Boot Menu, at startup, and change the Boot Order to make the DVD/CD drive 1st in the Boot Order, then reboot with the disk in the Drive.
At startup/power on you should see at the bottom of the screen either F2 or DELETE, to go into Bios/Setup, or F12 for the Boot Menu.
When you have changed that, insert that Bootable Disk you have made in the Drive, and reboot. http://www.bleepingcomputer.com/tutorials/tutorial148.html
Above link shows what the process looks like, and a how-to, as it loads to the Repair Options.
Neosmart contains the contents of the Windows Vista DVD\'s "recovery center," as we\'ve come to refer to it. It cannot be used to install or reinstall Windows Vista, and just serves as a Windows PE interface to recovering your PC. Technically, one could re-create this installation media with freely-downloadable media from Microsoft (namely the Microsoft WAIK kit, a multi-gigabyte download); but it\'s decent of Microsoft to make this available to Windows\' users who might not be capable of creating such a thing on their own.
Read all info at the website about creating and using it: http://neosmart.net/blog/2008/windows-vista-recovery-disc-download/
ISO Burner: http://www.snapfiles.com/get/active-isoburner.html
It makes a very good Vista Startup Repair Disk.
You can do a Startup Repair, System Restore, etc from it.
Note: It is not a reinstall disc.