Sometimes the length of an .avi file is recognised wrong by WMM. And since the time line editing is based on this info, the whole thing gets fucked up. The .avi is played correctly with Windows Media Player and other players, and the duration is displayed well in these players as well.
I use "CamStudio" to record stuff.

What could be the problem? How can I solve it? Is there a free and extremely simple video editor out there? All I need is time line editing of some .avi videos (I just want to concatenate some, trimming the beginnings and ends of the pieces)

Thanks in advance!

Windows Movie Maker is utterly incompetent at cropping and concatenating videos accurately and besides that there's not really much else it even attempts to do, especially in newer versions (since Vista I think) which have removed the majority of the functionality of older versions.

As for alternatives, you could try choosing one from this list.

AVI is a container format. Do you know the video encoding used? Some video encodings (eg. MPEG-2) have messed-up timing information, storing it all at the end of the entire video as a series of offset instead of something sensible.

Can you try saving your clip with a different video encoding within the AVI file?

It says "Microsoft Video 1"...

Yup, I tweaked the avi settings a bit, it seems to be okay now.