One of those terms was that they can change the terms at any time without notice.
Your fault for clicking "Agree". Didn't anyone watch the Human Cent-iPad episode of South Park?
" Unilateral modifications are not supposed to alter the material or important terms of the original contract. "
http://www.faircontracts.org/contract-provisions/unilateral-...
You have 72 hours (10 business days by mail) to undo your consent to the Windows license changes after clicking "Agree". Did you submit your notice in a timely fashion, or did you let the clock run out?