System Restore is a feature of Windows that takes regular snapshots of your machine. You can restore Windows to an earlier point in time with these snapshots, with the aim of restoring it to a time before you started experiencing problems.
System Restore will not delete any of your personal files or documents.
If you can boot into Windows either normally or via Safe Mode (see above), then you can run System Restore via Start -> All Programs -> Accessories -> System Tools -> System Restore.
Choose a date and restore point that you would like Windows to restore to. You should choose a date that is before the time you started experiencing the problems.
If you cannot boot into Windows, then do the following:
Windows Vista/7/8/10: Press F8 repeatedly at boot until you get the Windows startup menu. One of the entries in the menu is 'Repair your computer'. This may also be shown automatically as an option if Windows failed to start correctly last time around:
First, try running a 'Startup Repair' to fix any problems preventing Windows from starting up correctly (see image below).
Failing that, try running 'System Restore' to restore Windows to an earlier point in time (see image below)
Windows XP: Press F8 repeatedly at boot until you get the Windows startup menu. Choose 'Safe Mode with Command Prompt. You will need to type in a local administrator username and password. Then type in: %systemroot%\system32\restore\rstrui.exe and press return.