Click start -> All Programs -> Accessories -> System Tools -> System Restore.
If System Restore was turned of on your computer, you will see the following dialog :
If Windows shows you the System Restore wizard screen instead of the above image ...
... STOP ! You already have it enabled on your computer. Else ...
... click yes to enable it. Windows shows this screen :
Make sure to UN-check the check box that says "Turn of System Restore on all drives" and click ok.
That's all ! The next time you launch System Restore, you will see the "Welcome to System Restore" wizard.