I startup a computer and when I logon it says you must activate windows before loging on. When I attempt to activate it says it's already activated but then logs off...

I have XP SP1

You are going to have to call to Re-activate. This is the number for the USA
United States

(716) 871-2781

(888) 352-7140

It only takes a few minutes. A voice recognition bot will do all the work for you.

If microsoft say that its been used already; ie pass you through to someone, don't panic. Just explain that you had to re-install your machine and changed your motherboard.

Obviously, if you have installed this copy/key on multiple machines, your breaking the law, so be vigilant

On jan 1 i had to reactivate mine too ,legit copy ,because of so many hardware changes ,but mine went through