naisho: Taking a guess the route the OP took, he did a system restore or something along those lines.
use slmgr to reinput your valid key.
go to start, and go to the search box and type cmd to go to the command prompt.
input
slmgr -dli
for your Vista edition and status of the license.
Then if you want to reinput your key for reactivation, type
slmgr -ipk ##### - ##### - ##### - #####
where ## is the cdkey on the vista copy you bought from staples.
If you have not met the activation quota, you should be good to go. If you hit the quota limit you'll need to call microsoft licensing center at 888-352-7140 for the phone activation numbers. It's an automated computer that will handle your case by voice and inputs, you might need someone to help you out.