Recommended utility to fix and speed up Windows

Hi, Can you recommend a utility for dealing with all the errors and junk that build up in Windows over time, using up memory, disk space and slowing everything down? Most of all I want something trustworthy. The last thing I want is for a clean-up utility to break something, or to delete something that I need. I guess it therefore needs to have an undo capability. I would like to clean up...