XnView Software

I have a problem with too dark pictures and videos.

In your program there is such option as "Use Gamma Correction". I write there 3.00 and it shows pictures with normal quality! Ok.

May you explain what to do in order that video files (MPG, WMF, AVI, Real Video, etc.) look normally (not to dark) too?

The only way I can think of making this happen is to associate all your videos with FFDShow Video Decoder and then use it's "Picture properties" filter to get the desired result. You can download FFDShow here - http://www.free-codecs.com/download/FFDShow.htm