Well, you don't need to touch your registry for one..
As long as you have a decent graphics card you should be ok
www.doomsdayhq.com
You of course need the original WAD files for any games you want to use that the engine supports, and it does support a lot.. Doom, Doom 2, Ultimate Doom, Final Doom, Heretic, Hexen, prolly a few others.
Just be sure to tweak your graphics accordingly. It's a great engine, allows things that couldn't be done in the original like jumping, I think also bridges.. All kinds of nice lighting and particle effects. Playing it with all the details on really is a cool experience, even if its just 2D sprites.
There is also... Netplay
There are also graphics packs, for high resolution textures, 3D models of the monsters, etc.. Man now I wanna play again, haha