Strange issue.

My LapTop (Vista Professional) has 2 X 512 Mb Memory sticks. I replaced one of them with a 2Gb stick. On re-starting Windows it is now asking me to activate Windows. When I do, it says that activation has failed because the product key is in use, also, Windows Security Essentials tells me that I've failed the 'Genuine Windows Test' and will close in 30 days.

If I remove the 2Gb stick and put the old 512Mb back in I don't get the messages.

I have the original CD and Product Key (and they are both genuine)

Any ideas ?