|By default, recyclebin reserves 10% of drive capacity even though the bin itself may be empty. That size can be adjusted by rightclick>properties|
As mentioned above, System Restore also reserves 10% drive space for restore points. That can also be adjusted but if you reduce the folder size then there is less room for older restore points. In my opinion that's not necessarily a bad thing. I've never had a use for restore points older than a week.
Hey, look at that. 20% of 40gb = 8gb. How much space were you missing?
Goin' Fishin' (Some day)