System Restore is fairly straight-forward, as long as it is active (which it is by default)...
Go to the above list location and you should be able to select from any available dates and/or times that the computer took a mini-backup of itself... You should try to restore to a point in time before you started playing in the registry...
Oh, and obviously backup any important data - especially anything within C:\Documents And Settings\ (presuming you're running XP)..
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I've seen the light... It was green, flashy and attached to a Network Interface Card...
Whenever someone says "You can't miss it" I invariably do...