windows repair disk can even be a downloaded pirate version of windows7 or XP.
you dont need a serial key for it cause you already have that, just run the repair setup from the disk.
tons of ways to make an old computer work faster, people are always on the 'latest & greatest" hype, but I have several laptops that are 8, 9 years old that run like champions still.
I usually use those as loaners for when my family & friends ask me to work on their systems but dont wanna go without for the few days before i get it back to them.