Right click the folder that use to show as a zip folder.....where it says open with , select Zip and check the box as a default, and apply..
If Zip is not in the list, you may have to install a Zip program..
Since they open okay... there must be a ZIP program connected with the .zip format.
Check which program that is in the way Fran describes...
Then go to the execautable of that program in the folder where you installed it.. check properties of the program and see which pictogram is connected to that specific program...
You will probably find there is no pictogram selected, but the system allows you to set a new pictogram by selecting it ..