So why did you create a c partition of 900Gb in the first place? You should have create c with 150Gb at system install. Now you have to shrink the existing partition and that is limited by files spread over c. Try to run defrag. it might relocate some files, so you can shrink a bit more.
it's the first time i've ever partitioned, i didnt realise i'd be limited via windows way but never mind