Download and install the pre-configured packages ArenaSetup and DaggerfallSetup (just search the internet for these topics).

OK I found them. Do they inmstall the whole game again? Or can I just browse to my install folder? Thanks...

Both packages contain the full games. If you are familiar with the DOSBOX conf files, you can get them to use whatever has been installed on your PC previously - but what's the point of doing this? I'd say install the packages and copy your save files over (if you have them). Afterwards you can experiment with the conf files to your heart's desire (there are parameters affecting the graphics - you find detailed information for example on UESP.NET or in this forum).

Thanks I will check it out.