[QUESTION] Convert demo file to an other format video

How can I convert easily a video demo from Serious Sam SE to an other file format as for example avi, mp4 or mkv file
Thanks

I already made that kind of thread...You need to have Fraps of something,play demo and BOOM!You have avi...

DEM files are kinda outdated,there is no easy way to convert them...Trust me,i wasted 2 hourst trying to do so...