most of the critical windows file are hidden, you could also delete some restore point, turn off hibernation (that take about 21gb on my system), make sure the swap/paging file is on an HDD to clear some space or reduce it to the minimum
normally it should be 1.5times the RAM capacity but newer programs don't use a paging file anymore so in theory you could delete it but by caution i'd say, set it at 1gb like i've do...