Computers supplied pre loaded with Windows 7 hada special license for the OEM software installed To the best of my knowledge, those keys never expired, provided the major components, in particular the motherboard, were not changed. But, as you say this is self built, what does the the licence sticker relate to?
If you run Belarc or another key finder it will show a different key. The key for each manufacturer is known and there are anti piracy mechanisms in place to stop different manufacturer licenses being used on the wrong computer. You could try that key and see if it works