Sometimes when you make drastic hardware changes you will be required to reactivate Windows - http://windows.microsoft.com/en-US/windows-vista/Activating-Windows-frequently-asked-questions.
To do so, Open Windows Activation by clicking the Start button Picture of the Start button > clicking Computer > clicking Properties > and then clicking Click here to activate.
Keep us posted!
Any way to fix this? My Vista Home Premium is an OEM, and just recently it said "You're vista activation key is invalid" Etc etc. I entered my product key on the sticker, and then I got "You must restart your computer in order to advance." Then my start bar and icons were brought up and then I was told that "You may be using a pirated version. Certain features like the Aero color scheme etc etc. will not be usable." I assure you it's OEM, I payed for this two years ago, and I just recently reformatted with no problems. On a side note.
That's what I'm talking about.