Windows Vista Home Basic 32BIT reinstallation DVD

I have  Dimanesion E520, service tag DMFPK2J, ship date 31/10/2006, so out of warranty. I have had a doggy HD for some time but managed to limp along. A few weeks back it finally gave up the ghost. So, I bought and installed another. I used my Vista reinstallation disk ( P/N CT461) that came with the PC. (The operating system was already installed when I bought it, I didnt get a product key). Hey presto it seemed to work. Reinstalled all my other stuff on and it worked like a dream, for a couple of weeks. Then one day when I turned it on, it came up with "the windows vista home basic product key you typed is invalid for activation". Basically it tells be to buy a product key online. Do I reall need to do this? Is there a step I missed on teh reinstall? Can anyone help?

Hi, You should have a blue sticker stuck to your system with a number on it, that your product key. On the Vista and XP disc's, a factory produced number is on the disc, normally that does it, have you checked to see if you have a virus.