Did u you try booting in "Safe Mode"? If not, then try to boot your system in "Safe Mode". Check if yoir computer works fine in Safe Mode or not.
You can even try reinstalling the Video driver from http://www.dell.com/support/drivers...
If the issue still persists and If this issue started occurring after doing some recent software changes on your computer, then I would suggest you to restore your system back to a date when it was working fine. System Restore will affect Windows system files, programs, and registry settings only.
Hope this helps.
Thanks & Regards