Gabelvampir: DOSBox has a scanline scaler. To activate it start the DOSBox configurator/graphic mode setup, go to advanced settings, and there under scaling engine either choose scan2x or scan3x (whatever works better for your resolution). Oh and if you want to play in window with scanlines you need to change the window resolution setting to something else then original (something bigger then the game resolution), because the scaler is only used if the graphic output is really scaled.

I personally never liked scanlines even back in the day, but some people really seem to like them. To each their own I guess.

Edit: Looking over the scalers in the DOSBox wiki (http://www.dosbox.com/wiki/Scaler), you could also try the tv2x and 3x scaler, it also has scanlines and is supposed to look more CRT TV like).

MindsEyeSplinter: I found it - I failed to use that big, really visible link you put in your reply! Thanks again!

Gabelvampir: You don't even have to do it by hand, GOG has a config program for DOSBox they install with every DOSBox game. You can use the "Dosbox Configurator/Graphic Mode Setup" link from the Start menu folder of the game of the configurator option in GOG Galaxy.