You might also consider running a repair of the OS. This replaces all the system files with originals from the disk and lets you keep all your data and installed programs.
As they asked though, we need more info. Make, Model of the computer. Do you know if it has a restore partition? Do you have the original system and software disks that came with the PC? Also, do you have the COA with the product key? An XP CD is useless without a product key.
If you only have an old SP1 version of XP install disk, you can download sp2 free from MS. Then use free program called nlite to slipstream sp2 and any driver updates you need, to create a new bootable install disk. You can even make it an unattended install disk by specifying your time zone, language, screen resolution, etc before you create and burn the .iso image.
err... just saw recent post where SgtShultz said the same thing.