[Radek]

The Reset Adventures button will delete all data related to the current game run and return you to the beginning of the game. All your characters will be sitting in the Valeia Inn. Click the "Valeia Town" button after Reset Adventures button.
The Reset Adventrues action should not affect the W&W files, for example the .avi files, only the files related to a particular game run (current contents of shops, and so on). I am not an expert in it.
Your saved games are not lost after Reset Adventures. You can return to them whenever you want. Among others, a reason why your .avi files should be okay after Reset Adventures.