Problem with games and DirectX in Windows 10 Technical Preview

After installing Windows 10 Technical Preview (a clean install) pretty much 99% of my games stopped working. Most of these I got on Steam, so when I try to play one, it starts the first-time setup, where it tries to install DirectX and then it just returns to desktop. If I try again, it just does the same, but if it's one of the few games which don't install it when running them the first time (i.e., Binding of Isaac), then it has no problems.

After that, I tried to play a couple of relatively new games I got from GoG, but they gave me error messages when trying to install DirectX. They still install, but if I try to play them they don't run because they can't find DirectX-related dlls in the computer.

According to Dxdiag, though, DirectX 11 is installed. Reading forums, it seems to me that most people can play games with no problem with the Tech Preview, but some can't because Windows 10 doesn't recognize their graphic cards, and they have to switch to integrated graphics. I can't do this, as my motherboard doesn't have them, but at least when checking system information it seems that the OS recognizes the card (a Radeon Hd 4870, not exactly cutting edge stuff...), so not sure if it's the same, though it might be, as I tried installing the latest drivers for Windows 7/8 and couldn't. Has anybody had similar problems? And did anyone manage to solve them, and how?