Probably you messed up the files associations settings...
Open BSplayer, go to the Preferences window (CTRL+P) then select the "File types" section, select the AVI extension and the option "Register extensions on player starts".
Now restart BSplayer and then you should be able to open you avi files again
If the problem persists, go to the "Folder Options" window (open a window in Windows Explorer - not InternetExplorer - and select the menu item "Tools"), then select the "File types" tab, and there search for the AVI extension.
Once you find it, select it and press the delete button, then re-register the extension to BSplayer as I showed you above :)