Tune Up your PC for Free

Ashampoo WinOptimizer 2010, (otherwise paid) available for free from V3 Software Store is a top choice in this regard. 

You will need to register for the serial. WinOptimizer has a very sleek interface, with one-click cleaning for disk, privacy and registry cleaning tasks. It also has a nice option of a full scan, which gives recommendations for disabling unnecessary services, tweaking system settings, as well as the one-click cleaning features. 

However, WinOptimizer requires registration. Also, no further updates are provided. However, this is the best choice if you want something simple that can clean your computer with a few clicks.

It also has a tweaking utility with over 450 tweaks.