It depends on what instructions you are asking for.
Can you access the XP file system from your Linux boot?
I can't tell you how to use Linux to do file copies -- I have to assume you know that -- the only Linux experience I have is a puppy linux boot disk and I found the file copying procedure rather non intuitive.
The basics go like this, if you can access c:\windows\system32\config then you want to BACKUP the system, sam, software, security and default files in that location to a backup folder you create. This ensures that you can atleast restore them if that proves desirable.
After you have backed them up you navigate to c:\windows\repair and copy the same files from the "repair" folder to the c:\windows\system32\config folder and overwrite the existing files.
Now you would need to reboot; if the reboot is successful you can then follow the rest of the MICROSOFT instructions to complete a System Restore to a more recent time -- which may restore the functionality of installed programs whose registry information is not going to be contained in "repair" folder hives.