take some time to read the beginner's guide...
http://forum.ccleaner.com/index.php?showtopic=1504from the beginner's guideMenu Order CacheThe Start menu allows you to set a custom order to the programs listed, although there's no option to sort all folders by name. Selecting this option removes any custom ordering from the Start menu.
Tray Notifications CacheIn Windows XP the System Tray (the set of icons next to the clock in the bottom-right corner) orders and hides items automatically for you. Whilst this is useful, it records a list of every program that has been run from the tray. This option clears the list of previous programs and removes any custom display/hide options. (Warning: This won't have any affect until you manually restart the explorer process from the Task Manager.)
Window Size/Location CacheWindows Explorer stores the view formatting and ordering settings for each folder on your system. After some time, this can contain a large amount of redundant data and may even slow down browsing for files on your system. Selecting this option will clear this data and reset the Windows Explorer display settings to their default option.
User Assist HistoryThe start menu on Windows XP displays a list of the most recently run programs. Selecting this option will clear this list.
IIS Log FilesIIS (Internet Information Server) is the service Windows uses to display web pages. This comes with most XP Pro and Windows 2000 installations and is generally only used by web developers or designers. Every time someone requests a page it gets logged to a text file. Unless the computer is a web server, these are generally not needed, and may be safely deleted. This option will safely delete these files.
Custom Files and FoldersWill clean your custom files and Folders that you have selected in Options, Customs.
there is one more
Hotfix Uninstallers... which remove the uninstallers of Windows Hotfix from your system (does it make sense to explain it this way...?)