Here are some guides to help you carry out a System Restore in Windows.
Windows ME -Click here
Windows XP - Click here
Windows Vista - Click here
Windows 7 - Click here
In cases where you have trouble logging on to your computer normally (normal mode), you can make use of the Windows Safe Mode instead:
Simply restart your computer, keep pressing the F8 key on your keyboard while booting up and it should offer an option to load Windows in the Safe Mode. Once you logon to the Safe Mode, you may proceed to successfully restore your system using the System Restore feature explained above.
Good luck with the restore!