to enter the registry hit the start menu, select run then type regedit this will take you into the registry editor... which show you a working of the pc configuration. The registry editor has a search facility which you can use to look for specific registry keys such as photoshop's however there will be a large number of these.
As correctly stated messing up the registry can stop your PC from working.

Registry cleaners will not help you in this situation simply because the key is neither unrequired or damaged which means to remove it you have to do it by hand

What version of photoshop you got ?
If its the same as mine ill show you exactly where to look