Hoe kan ik een filmpje gemaakt met iMovie afspelen op Windows?

Your movie is ProRes, that is the problem.

ProRes is a high quality codec that is adequate for editing.

Most people do NOT have the ProRes codecs installed, so they cannot play this.

You should export your movies with the H264 codec for greater compatibility (with just about any device, as I wrote before).

You can convert this directly in the Finder, if you like.

Control-click the file, choose Services->Encode Selected Video Files... and choose H.264 with the appropriate frame size (1080p in this case).

What version of windows? How did you export from iMovie?

The most usual format would a mov file with H264 codec, which just about any computer released in the last ten years or so should be able to play.

Select the file you exported from iMovie, open in Quicktime Player, press command-I and post a screenshot of the Movie Inspector.

EDIT: I just tried a simple File->Share->File... and it produced an mp4 file with H264 codec.

That is just about the most universal format there is.

It should play in any mac, windows computer, modern phone or tv with usb input...