If they have an actual XP CD, then a Repair Install, which leaves all the current user's data intact, should correct installer errors (it is one of the suggested repairs for this). But I am surprised we have not arrived at a point where the scan logs provide the needed info. A Repair Install is basically the same as doing an upgrade, but in this case it is upgrading XP to XP, and returns most of the default Windows files and settings. The install will return to the system to whatever level of Service Pack is on the CD, so all upgrades and patches will have to be reinstalled after.Here
are some steps for a Repair Install, and if you have any questions after reviewing them be sure to ask.