Some people do periodic clean installs to restore XP speed. XP "speeds up" because clean installs creates a new registry free of all the programs and settings you had before. If you choose to do that, backup the files/folders you want to keep, preferably on another HDD (either internal or external), so you can move them back after you do a clean install. If you leave them on the existing XP HDD, the clean install will "lose" them when XP creates a new Master Boot Record (MBR) using the NTFS format or a new File Allocation Table (FAT) for a FAT file system. Further, it will allow those files and folders to be over-written by both the XP install files and any other programs and drivers you install after the clean XP install. If you do a clean install of XPsp3, plan on downloading and installing ~100 updates from windowsupdate.
You will also need the disks for programs you want to re-install, e.g., office suites, media-editing programs, etc.
An alternative is to use discipline in installing programs - only install those you actually use. And to use anti-vrus and firewall programs that you keep updated. And stay away from suspicious websites and email attachments.