Depends on if you did a repair install, or a fresh reformat install. Which one?
If you did the repair install, put the disk back in and repair again (do not reformat) Activate the product over the phone, not the internet. Unless you just have an invalid key code.
If you did a fresh reformat, there is no way to recover an older installation. Repeat the install if needed, activate over the phone.
If this fails, get a fresh XP SP3 disk, repair install and activate.
Hopefully you did not reformat, or you have lost your personal files.