Decided to have a stab at this and it seems to have worked. So here's how I installed Windows 10 on a ten year old Pentium 4 with 'unsupported' NVidia GeForce 6200 Turbocache. (Upgrade from Windows 8.1).
1. Backup. To minimise later regret I mirrored my whole c: drive onto a spare hard drive with Acronis True Image.
2. Using Windows 10 Media Creation Tool (http://windows.microsoft.com/en-gb/windows-10/media-creation-tool-install) create a Windows 10 DVD image and burn to DVD.
3. Download latest NVidia driver 309.08-desktop-win8-win7-winvista-32bit-international-whql.exe from NVidia and save the .exe file for later use.
4. Install Windows 10 using the Windows 10 DVD.
Installation went fine, but as expected, all Nvidia Drivers were lost and the PC ended up configured with the MS Basic Graphics Adapter (and incorrect screen resolution).
5. Once Windows 10 is set up, fully configured and bedded in, run 309.08-desktop-win8-win7-winvista-32bit-international-whql.exe. Officially Windows 10 is not supported, but the installation ran without a hitch (Windows 8 is supported - they must be similar enough), and I have checked in control panel that GeForce 6200 TC now has drivers dated 30/01/2015 version 9.18.13.908, which is as expected. The result looks fine.
All seems to be well. The automatic Windows 10 update would not even start because it claimed the 6200 is not supported (officially, it isn't), but everything now seems fine with the Windows 8 driver after pushing through the Windows 10 upgrade with the DVD. In non-gaming use this 10 year old system (admittedly with Windows installed on a SSD) runs as fast as my quad-core i7 rig.
Might not work with other systems but it certainly worked for me. Good luck, and don't forget that backup...