Hello Raphael,
Noel emailed me to see if I had anything that might help out :-)
My understand of your situation is that your computer had a corrupt system file, so you used the manufacturer's recovery process to recover your computer to "out-of-the-box" condition. I also noted that you mentioned the recovery gave you the option ot have your personal files saved in a place called "My Old File Structures." Unfortunately, the recovery did not go 100% smoothly and you are experiencing an activation glitch.
The best trick I have up my sleeve is to try to "rearm" Windows XP Product Activation, which if it works will provide you with a 30 day grace period, during which time you can attempt to recover and backup your files from the My Old File Structure, offload them onto other storage (such as external hard disk drive or flash drives). Once your data is safe, then you can try yet another recovery and hope that goes more smoothly this time. If not, your next step would be to contact your computer manufacturer to acquire the system discs needed to run the recovery from CD/DVD rather than from the recovery partition on the hard drive.
If the computer can be started to a safe mode command prompt, run this to re-arm XP activation:
rundll32.exe syssetup,SetupOobeBnk
The command must be typed exacly as above with space, caps, and comma. Restart, and if it works you will be able to log on in normal mode (as opposed to safe mode) and will be in the 30-day activation grace period.
Assuming this works, rescue your data, then retry the recovery. You will want to visit the support pages and community forum hosted by your computer manufacturer to research and see if recovery glitches are common for your make and model of computer--if so, there is likely to be a fix for it that you can implement.
If it does not work, then your next task is to use an offline operating system such as BartPE, Ultimate Boot CD, Knoppix, etc, to rescue your data from that saved location. Once the data is off the computer and safe, then you can use the manufacturer's recovery discs (or a generic Microsoft systembuilder/OEM disc) to do a good clean installation of Windows.
Good Luck! :-)