Disk Space

I have a Dell PE2950 with a Raid 5 using 4 250GB Sata2 drives on a server 2003 standard R2 operating system. Windows shows the disk with 188GB of free space but if i highlight everything on the drive and check the properties there is only 150GB being used the total space on the drive is 684GB. Any idea what might cause the drives to read fuller than they are? Any help would be greatly appreciated.
Thanks,
Jonathon