Martin,
When an auth code is entered (for standalone licensed products), license files are created in the folder C:\C_DILLA, which is hidden to prevent accidental modification or data loss, either of which will force immediate reauthorization, as you have found. Both max and AutoCAD's licenses are here.
Error [4] indicates that the license files existed previously, but were deleted. This can happen either by direct manual action, or by manipulation of files outside the OS, such as by restoring partition images as you have done.
There should be no problem installing max 4.3 on top of AutoCAD 2000i. They share a common component, the licensing drivers (called the C-Dilla RTS).
My suggestion is to stay with your system the way it is now (with AutoCAD working), and get your 3ds max auth code (by the way, there is an option to get auth codes over the web, 24x7, in most parts of the world). Don't modify system32\drivers, and don't restore any partition images.
The licensing drivers that come with AutoCAD 2000i are good enough for Win2K, so you don't need to install RTS 3.27, the latest release. This won't solve your error, though, which is caused by the license data files being deleted, not by software.
There are many licensing technical documents available in both the Autodesk Knowledge Base as well as on the Discreet site (support.discreet.com).
--Cy--
autodesk