how do i record a demo with doom etc.

and does anyone know how to convert them into avis, movs or something like that. any help would be appreciated.

doom -record demoname

Add the -maxdemo parameter (plus the size in kilobytes) if you're recording anything longer than 15 minutes. I think the default is 256. Like:

doom -record demo01 -maxdemo 1024

You can add any applicable additional parameters, of course (and all this works with any DOOM source engine, not just the standard doom.)

There's an app for making demo MPGs, but you have to compile it (I don't know if compiled binaries can be found somewhere.) These MPGs lack sound, though.

http://www.physik.uni-leipzig.de/~girlich/games/lmp2mpeg/

there are 2 hacks of Prboom that output bmp files as a demo plays.
there are many tools to convert those images into a movie.
the trick is routing the sound output.
I would look at the Quake movies (not quake2 or 3) and how they captured the sound.
gl.