I built my computer about a month or two ago and I re-used the same harddrive as I had on my laptop and now it says my Windows7 is not genuine, is there a way to fix this besides slmgr -rearm or buying Win7 again?
If the copy of win 7 came with the laptop it's OEM and OEM copies of 7 are tied to the motherboard they are first installed on. So most likely you need to buy another copy.
These things hardly ever work but your computer started up, that's major luck. Now, the original Windows is paired with the motherboard and if your HDD is moved/changed/formatted and re-installed, your PC won't be original. So all you have to do now is to get another version of Windows or postpone the activation.
(Go to services, end the "Windows Activation Service" and your PC will be fine for one month more!)