Try running ScanDisk (right click on the c: drive in Explorer, select Properties, then the Tools tab, then the Check Now button under Error-checking Status - this should be faily close for most versions of Windows). Especially for FAT32 (Win98/ME, sometimes XP), the free disk space can often be wrong after moving or deleting a lot of files (Windows caches the free space to update later, but sometimes never gets around to it).