CCleaner

Been really happy with this and I think you will too. It's free:

CCleaner is a system optimization and privacy tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It's fast (normally taking less that a second to run). It cleans: Internet Explorer (Temporary files, URL history, cookies, Autocomplete form history, index.dat); Firefox (Temporary files, URL history, cookies, download history); Windows (Recycle Bin, Recent Documents, Temporary files and Log files); Registry cleaner (Advanced features to remove unused and old entries, including File Extensions, ActiveX Controls, ClassIDs, ProgIDs, Uninstallers, Shared DLLs, Fonts, Help Files, Application Paths, Icons, Invalid Shortcuts. So comes with a comprehensive backup feature.) Third-party applications (Removes temp files and recent file lists (MRUs) from many apps including Opera, Media Player, eMule, Kazaa, Google Toolbar, Netscape, MS Office, Nero, Adobe Acrobat, WinRAR, WinAce, WinZip).

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