Downloading trouble

I have been trying to download files to my computer lately and it asks me if i want to "open or save the file". it never used to ask me this before and now if i select "open" it asks me with what program and i dont know. and if i press "save" it downloads but not as an openabla file- my computer says it was not able to find the file and open it and if i want to go online to find a program to open it.

what is going on- i used to be able to download anything i wanted just fine :(
i have windows vista ultimate