Actually, In Windows some space is devoted to recycle bin & system restore(if U r on XP) from each & every partition .. defaut is 10% for recycle bin & 12% for System Restore .. so dunt worry! ..
Just to get back ur space make recycle bin! space to
zero .... & turn off the system restore settings for once then check again wat U checked before!

U'll see the correct & accurate results probably! ...
& forgot to tell ya

.. from the windows installed partition .... if U keep
hibernation ON! then tht also takes some space from tht partition! .. also, ur virtual memory! takes some space but it doesn't affect much!
Try out wat all have been suggested! U'll get back ur space
safe .. gud luck! & relax!