Power the computer off and then back on. When you see the machine logo (HP, Dell, whatever) begin pressing the F8 key repeatedly until you get the advanced boot menu and then choose Safe Mode. Once in Windows, implement a restore point.
That won't work if you have Windows 8, however. If you have Win 8, search for instructions on booting safe mode. Microsoft in their infinite wisdom decided to make it a real pain in the ass in Windows 8.