How do i activate my computer?

It's a Dell Inspiron 8100, Windows XP Home Edition 1985-2001 and it's an old laptop ( like 2000-1) and it had a nasty virus, but i dont have the original disk, so i download a disk for it. Now when it comes to activation, i cant. I found a video that tells me to go to the Registry Editor and go to Software and so on, leading to changing some numbers and it DOES work, but only up until i restart it. Please help!! The video is: http://www.youtube.com/watch?v=KllnJ1J7eio