Cleaning Hard Drive

I have been installing and uninstalling software on my computer, by using the add/remove option in Windows Xp Pro.

Can anyone tell me of a software that will clear all the files left behind (Even in the registry) when i have uninstalled the software, so to make my computer run quicker and smoother.

When i look find what programs start up ( by using run and then msconfig) i notice that there is still some software in there that i have uninstalled how can i remove these.

Which is the best defrager.