It depends on what type of reinstall you do. If you do a clean install, then some files will still be left over from your previous install unless you allow the installer to format the partition before it installs. If you do a repair install, the installer will keep the majority if your programs/files intact. Your user accounts and files will be preserved, but any updates/patches you applied to Windows will need to be reapplied. To do a repair install, at the windows boot up installer, press enter to install XP. It will then detect a previous installation, and by pressing R you will repair the existing install.
