I have a Compaq Presario running Windows 7. Yesterday it downloaded several updates from Microsoft. Now when I start it, it goes to the log on screen and says "The group policy client failed the login. Access is denied". And that is it...can't get further than that in safe mode either. I click ok, it says logging off, and turns off. AAAARRRGGHHH!!
Phil, Robin's System Restore idea is a good one but you'll obviously have problems running it if you can't login.
Fortunately, most PCs, and especially big-name brands, have a "Repair your computer" option that includes the System Restore component. You get to it by starting your PC and immediately tapping F8 quickly until the menu appears - the option you need is at the top. Launch System Restore and choose an available restore point that pre-dates the start of your problem.
Note: If available as a repair option, don't click on "System Recovery" (or similar sounding option) as that restores the PC back to its factory defaults and erases all data.