Before you attempt an XP or Vista installation, back up all your C drive data. A clean install will reformat the C drive, erasing everything, including applications not included with the XP or Vista installation. To perform a clean install, insert the XP or Vista CD-ROM or DVD into the CD tray and restart the computer. This will boot your computer from the CD. When the computer displays a list of partitions, choose the C drive (where XP is currently installed) and select Format. The CD will perform a fresh installation.