You will need to activate Vista - only when you do a clean install is it pre-activated.  You can try the online activation  (see below) but it will probably fail - in which case you'll have to call Microsoft and explain what you have done. [URL]
I recently added 2 gig memory, a new processor and a new hard drive to my Dell XPS 420. I used my "(Dell) Reinstallation DVD" to install Vista to the new hard drive which is now C:.  The original Vista is still on the old hard drive E:. FWIW, during the new HD install I sometimes booted the PC from the new drive and sometimes from the old drive.At some point, Vista put that "Evaluation Copy" at the bottom right of the screen on the new hard drive Vista.  Also, again at some point, Vista asked me to validate, which I did without any problems. Anyway, my question is how to make the ...Evaluation copy... notice go away?  Many thanks,Terry
New hard drive, now Vista says "Evaluation Copy, Build 6002."