I also have GeForce 6200 TurboCache. Drivers are dated 30/01/2015 version 188.8.131.528, which I believe is the most recent. However Windows 10 upgrade will not run, saying "The display manufacturer hasn't made your display compatible with Windows 10. Check with the manufacturer for support."
If I run a windows driver update through control panel (instead of using the NVidia tool) it simply rolls back the drivers to 07/12/2012 v184.108.40.2068, which doesn't help at all.
I have read that if you force the upgrade with the media creation tool you are left with the basic microsoft graphics adapter, so I'm not keen to try that approach.
Has anyone managed to upgrade a PC with GeForce 6200 TurboCache to Windows 10, and if so, what is the workaround please?
If there is anyone out there with GeForce 6200 TurboCache who has forced the upgrade to Windows 10 via the media creation tool (or any other way) and is then faced with the MS Basic Graphics Adapter, you could try manually installing the most recent (Jan 2015) Windows 8 driver (309.08-desktop-win8-win7-winvista-32bit-international-whql.exe (174,641KB)) from NVidia, which I can confirm worked with no issues at all in my Windows 8.1 setup with GeForce 6200 TC. Don't know whether it will work or not with Windows 10, but if it does please report back here!
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 220.127.116.118, 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...