Sarah - yes it will. A fresh installation of XP requires you to format the disk so you can install on the fresh partition. If you don't format it will install it on a new partition but that will leave you with two operating systems on the disk...

You will need to format and re-install if you want a fresh installation, and that will wipe your files. so you are going to need to back up these files to CD-R, DVD-R, USB HDD, or a second IDE HDD (if you have one).