depends of what you want to do, an example if you want to delet a software registry do this - Open the regedit thing - go to HKKEY_CURRENT_USER and open the + simbol - there will be a Software folder there open it and all your installed softwares registers will be there - click the one you like - and delet the files inside or just delet the entire folder (the software you want) entirely. after this there will be no registry of it anymore man. if the explanation isnt good i make a better one with more time, just ask if you still need
--- Small update ---
i forgot something the HKKEY_LOCAL_MACHINE folder also has the softwares folder (if its a software instaled for all users you must delet there aswell. also if it has more than one user and adm you have to do it with all other users and not only the current user folder.