Do you have the license that came with the Dell for Windows XP ?
If yes you need to download an OEM ISO image of the XP bootable CD.
You will also need to download all drivers for XP for that particular model DELL.
It may sound like a pain, but once you got all that, you will have complete control over installing, uninstalling OS. Microsoft should or can help, if you need a valid SP3 copy of XP, just tell them what happened and provide the license no.. You can fix it by copying the new "good" folder, but I would do a virus/malware check on boot first. Second problem is you need to switch your windows booting to the old directory. Unless it gives you two windows versions on boot.