Just backing up the registry or using the Windows System Restore is not enough. System Restore backs up system files of certain extensions (.exe, .dll, etc.). You need to take a full snapshot of your PC using an imaging/cloning program like Norton Ghost. There are some free alternatives to Norton's Ghost:
Clonezilla: Clonezilla is an open source solution to computer backup, restoration and "ghosting". It is based on partition imaging and allows you to backup and recover your system should it be taken out by a virus or a hard drive crash.
FOG: FOG is a Linux-based, free and open source computer imaging solution for Windows XP, Vista and 7 that ties together a few open-source tools with a php-based web interface.
Using one of these programs, you can take the full snapshot (incl. all files, programs, settings, registry) of your PC and go back to that moment anytime you want.