That will depend on how thorough you are. The overhead of orphan key is tiny, it won't really matter unless the machine is under spec to the point of unusable to begin with. It's not too different from having some left over files on the harddisk. What can really adversely affect performance are left over components like drivers, services, registered COMs, shell handlers, etc. They are however hard to identify unless you log the installation process in the first place. Even then, there's the danger of cross dependency. I have yet to see any registry cleaner or 3rd party uninstaller that do a good job of it and it's hard to imagine someone putting the enormous effort to do it manually. If you are the kind of person that like to try out different programs, I will recommend you do it in a virtual machine and only install those that you really like in your normal windows. You can use Windows in trial mode for 120 days, so you don't really need to activate the copy in the VM.