The easiest way to do it is to totaly reformat the harddisk and than do a fresh install of the operating system. Make sure you backup the necessary drivers on a CD(if possible) If not it will be somewhat more time consuming to do it.
1) Uninstall all programs you do not want to give along
2) Reboot and Download install and update Ccleaner
After installing and updating go to options - advanced and tick prompt for registry back ups
Now Start up in safe mode
Do two runs
a) Click on problems and tick all. Click on analyse. At the end you will be prompted for storing a backup.
This will be stored, as a default, in My documents.
Click on cleaner click on analyse and accept all files to be removed.
Clean out the "My documents"folder as well as any other folder yo feel sensitive data is in