When you computer is loading press and click F8 continuously until a screen appears full of white text and a black background appears aka boot menu. Select "Repair Windows" (I think thats what it is called) if there is not that option go into Safe Mode.
How to fix:
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If you choose Repair Windows then a screen will appear for all sorts of options, choose System Restore. Now follow the wizard and you will now have a choice where you want to restore it to. Select the day before it stopped working. (It is usually caused by Windows Update, so if Windows Update is available I'd personally choose it, when you restart your computer, boot it like normal and hopefully it worked.
If you choose Safe Mode you will need to wait till your computer boots and access your account. Then go to Start Menu > All Programs > Accessories > System Tools > System Restore. Now follow the wizard and you will now have a choice where you want to restore it to. Select the day before it stopped working. (It is usually caused by Windows Update, so if Windows Update is available I'd personally choose it, when you restart your computer, boot it like normal and hopefully it worked.
Good luck. (Y)