How to use Winsplit:
1. Open Winsplit

2. Make sure to have "join" checked !

3. Now click "Source file" and choose the file with the ".1" format:

4. Finally, click the "Join" button and wait a few seconds/minutes.
It will automatically join the right files together and put the wonderful .avi
file into that folder where your source file was.
Now, you should be able to watch it !