We use Diskeeper.

Yes, I'm sure that the defragmentation API is used since every defragmentation programm has to use this API (Microsoft doesn't allow any other way). This was always like this in any WinNT version.

In WinNT 3.1 and 3.5 the API didn't yet exist, so Executive Software (Diskeeper) had to buy the WinNT source code in order to implement their own defragmentation functions. All Diskeeper users had to get all updates and Service Packs directly from Executive Software since the Microsoft files where incompatible with their version of Windows.

Starting with WinNT 4.0 Microsoft implemented a few APIs for defragmentation in Windows. Since this time all defragmentation utilities have to use this API.