How to stop those security warnings?

I have encountered a lot of "open-file security warnings" lately. Whenevr I rtried to open a program, a window would pop-up and asks me if I really wanto run this program. I can either say " run" or " cancel". They are really annoying and I have no idea how they suddenly appeared. Any way to stop them or answer them only once and be done with it? Please help!

 

what OS are you running? also are these popups when you try to open it on your computer or in a download dialog box? (i.e. download complete, open open containing folder cancel)

 

I have an XP professional with SP2. Those pop ups show up when I click an icon to open a program, say, Norton antivirus. They even show up for Microsoft's own program; e.g. "help & support"; call it from " unknow publisher". It wasn't like this before. Only happened recently. Would you be able to shed more light on this issue?