The Microsoft Systems Management Server 2003 (SMS 2003) Operating System Deployment Feature Pack (OSD) uses Windows Imaging Format (WIM), an imaging format that Microsoft actually developed for Longhorn Server and Windows Vista and which is a different imaging format than that which Automated Deployment Services (ADS) uses. (For more information about ADS, see "Automated Deployment Services," February 2004, InstantDoc ID 41398.) WIM is a file-based, rather than sector-based, image format (sector-based files are stored in a cabinet-file-type structure. Because it's file-based, a WIM file can be restored nondestructively—that is, the target disk doesn't need to be empty and any existing content is maintained—and WIM supports variable-size target drives.
Additionally, WIM's file-based format also allows single-instance storage of files. This means that if a file and its SHA-1 hash-file "fingerprint" are identical, the file needs to be stored only once in the WIM. This might not seem important initially, but remember that Windows 2000 and later have Windows File Protection (WFP), which caches copies of core system files in a protected area from which they can be restored should the working versions become corrupted. Therefore, most system files (e.g., .sys, .exe, .dll) are stored twice, whereas they're stored only once in the WIM file. WIM also uses compression technologies, which, in combination with single-instance storage, reduce the image file to around one-third the size of the data being captured to the image—a better ratio than a sector-based image format offers.