minecraft exe not running

when my son try to logon to Minecraft horse updates (google seach)
https://mojang.com/2013/07/minecraft-the-horse-update/

for Minecraft - when he select the windows version - instead of "run" option he now gets a message....

"do you want tio save microsoft_exe(600KB) from s3amazon.com" and then a save or cancel option

the web page then freezes...

It was working fine on Sat 11th Jan - but in the afternoon - this fault developed and we still can't seem to find a solution.

We've checked java, its security options ....

please help with advice !