I cannot set the correct size for a movie 320x240. If I stretch the window it reveals only black. Same thing if I want to move window. How to set? No problems with mediaplayer...

Update the drivers for your graphic card.

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Thanks, but for this old S3 there is no more driver updates.

S3 Trio 3D2X or older S3 Trio64 ?

Yep S3 Trio 3D2X. My Canon avis play very well I think that some other files were corrupted.

Nice videocard, good 2D quality at it's time.
Try to install an alternate mpeg decoder such as Elecard or BitCtrl if the movies are mpeg ( I suppose)
If not, try to swich in video->renderering options to overlay mode 2.