While Windows Disk Cleaner does probably a quite good job at removing temporary files, there are probably more non-Microsoft programs on your computer that have left temporary files somewhere on your computer. Also, every user tends to install, upgrade and uninstall software and this might leave many unnecessary entries in Windows Registry - a place where all information about hardware (monitor, sound card, video card, mouse, etc), software and user settings is kept. When registry contains entries to programs and files that do not exist anymore, Windows might start slower or display errors.
CCleaner (also known as Crap Cleaner) is a free program for cleaning up such temporary files and unneeded settings in Windows registry.
Downloading and installing CCleaner
First, go to CCleaner download page and click Piriform.com in the CCleaner Free section.
After downloading and launching the setup file, CCleaner Setup Wizard begins with asking for the language to use. English is fine, so click OK here:
Another wizard with loads of Next-buttons! Don't we just love those things? 
Click Next:
And another License Agreement with I Agree button to click on:
Clear the Add Desktop Shortcut box and click Install:
You might or might not see an offer to install some other software after clicking the Install button! In my case, it has randomly been Google Toolbar. Do not install any software you do not really know about or need. The word "Free" means nothing useful in such cases!
Clear the Install the free Google Toolbar along with CCleaner box. Then click Install:
Here's another example of CCleaner offering other software. Clear both Include Google Chrome, along with CCleaner and Make Google Chrome my default browser boxes and then click Install.
CCleaner will be installed.
If you want to run CCleaner right away, leave the Run CCleaner check box selected.
Clear the View Release notes box.
Click Finish to exit setup process:
Updating CCleaner
As CCleaner checks for updates automatically, a new version notification window appears from time to time when you launch CCleaner. Click Yes to download the new version.
Internet Explorer will open, find an click on Click here to download link.
The rest goes exactly as in downloading and installing part.
