The standard method of Windows for cleaning private data is not reliable, you must use a tool like Tiny Shredder.
Tiny Shredder is a tool for removing a folder or of file from your hard disk. In a Windows Explorer-like interface, it prompts you to localize your file or folder and to click to erase it completely. Tiny Shredder makes it impossible to recover the deleted data with a recovery software because it overwrites many times the content of your file before deleting it. At the same time, it also deletes temporary files, cookies, the history of the visited URLs, the list of recent documents and even files in the recycle bin to protect your privacy.