Sorry for the long subject, but I tried to explain the problem there. =)

Anyway, I often seem to push something, no idea what, but that something causes me trouble.

Everytime I push the 'play' button below the sequence monitor window (I mean the right one), a white frame blinks a few times inside the monitor, and even though the sound plays - the video is frozen. Also, the video quality is a lot worse.

So what is this thing I'm doing, and how do I revert back?

Cheers for the help, whoever provides it.

 EDIT: Silly me. I posted this and only then tried to search for it. Sorry.

 First, make sure that no tool or bin windows are in any way overlapping the composer monitors window. Also, click on the composer window and go to the windows pull down and choose "home" to restore default position. Do the same for the timeline window.

For quality, look on the timeline toolbar and set the quality (screen) icon to full green, if it's now yellow/green or yellow.

Cheers for the help, found about the home button using search, but the quality is definitely new to me. Thanks a lot.

 You're quite welcome. Glad that worked out for you.