Ok, after I reinstalled Windows XP Media Center Edition 2005, I was required to enter a product key. It worked, and reinstallation continued. When I get to the login screen, I type in my password and it tells me "Before you log on, we must activate windows." I was presented with 3 choices, activate online, activate over phone, or don't activate now. I chose activate online. It instantly brought me to a window that told me my product key was invalid. I re-entered the product key that was on a sticker on the top of my PC. It doesn't work. It worked before, when I had to enter the product key at the beginning of reinstallation. I re-entered, over and over and over again. STILL did not work. I kept getting the same message: "Incorrect Product Key." Please assist me in ANY way you can, I will be VERY thankful.