Windows saves your preferred program to open different types of files like text (.txt are opened by notebook), images (.jpg, .gif. etc.), .pdf (by Acrobat), or spreadsheets (.xls by Excel). If you try to open a file that does not have a program preference set, you should be prompted for which program to use.
I wonder if you've accidentally set a preference that is not compatible with that type of file. You might try opening a program that you know should work (like GIMP for a .jpg) then use [File][Open] to open the downloaded file. If it's still jibberish then that file may have been corrupted during download. If it opens OK then try to reset your preferred program for that file type.