@maormini: Nic was right to yell at you, you really shouldn't have posted a question like this.
Now, about the problem: it was xvid's fault. The bug (in vfw code) was spotted only a few weeks ago by Cyrius of virtualdubmod team. It's fixed now, but I can't confirm if newest Koepi's build is new enough.
How to join them: remux both files to ogm using any tool you want (virtualdubmod for example). Then, join them using virtualdubmod and save again (to avi, if you want to). Perhaps you can also use matroska for the intermediate file, I don't know.
This problem has been around for a long time, all xvid files have it
Radek