Can't activate after replacing hardware

Hi

I have just this afternoon completed the replacement of my husband's mobo, CPU and memory. I've tried activating the pc although I had to change the password but that was eventually completed satifactorily. I then tried activating Windows and was told they couldn't reactivate it. At first it came up with a message to the effect that the servers were unavailable and then when they were available I clicked on "This is the device I am using right now" but still it said it couldn't activate Windows. I followed microsoft's own instructions to the letter but still I am locked out. So my question is: what am I doing wrong?

Any help would be very much appreciated Thanks.

Tracey