Virus Trustworthy Source Warning using Action Buttons

[ST]

when using powerpoint's action buttons (they are hyperlinked), i get a
message stating:

"opening [filename] some files can contain viruses or otherwise be
harmful to your computer. you should be certain that this file is from a
trustworthy source. would you like to open this file? yes or no?"

how can I get rid of this message and allow the button to hyperlink to the
file associated with it?

thanks

ST