Do not mess with the registry if you don't know what your doing!!! ALWAYS make a back up first.
I use CCleaner click here its safe on default settings and/ or RegscrubXP click here
Cleaning the registry is not really necessary unless you have
a virus or malware to remove (the anti virus / malware programs will do it for you)
notice a device problem (CD/DVD drives won't read)
your machine is slowing down badly
our are constantly installing and removing trial software (see slowing down)