I can't download anything on my PC.

Hello,
I have a Windows Vista and from couple of months i can't download anything on my PC. I have no idea what happend, when i download something and the bit that says "open" or "save" comes on i'll try to do both but non works after i press them nothing happens and none of my firends knows what is wrong with it. I'll be really thank full if somone will answer my question as soon as possible.