i am using win XP and secondly i know 1000!=1024 but that does not mean i wil loose 600 mb and one more thing it'a actualy a 4.3 GB hdd windows showing as 4.00 GB already making up for 1mb=1024kb
Aside from making sure there aren't any massive stashes of temp files or crud in your recycle bin, see how large your pagefile is. Windows has a nasty habit of not adding it into used space calculations in certain circumstances.