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Most Windows users discover it with time: right after Windows is installed, or when one gets a new PC, the machine is "clean". Startup and shutdown are fast, the system directories are not crowded with files that contain God knows what, no useless program is wasting your CPU 24/24, etc.
Then, things start deteriorating...

Windows does not provide an easy way to keep its system clean.
For example, there are many different ways for a program to install itself so that it is run automatically, so it is not easy for you to track down which application decided "by itself" to use your system's resources in the way it wants.

Slowly, your machine becomes a mess. Your CPU is wasted by running processes that you don't need. Applications create files in your system directories and leave them behind. You sometimes refrain to install and evaluate a new program because you are afraid that it will not uninstall everything afterward. Anyway, the time it takes you to start and stop Windows become longer and longer. You start thinking about reinstalling "just to get things clean again"...

More Info: Tiny Watcher