Your computer is infected with virus which is preventing you to download . At first restore your browser settings to default and then give a try. If it works then ok else you need to install a good antivirus to scan your system for getting rid of this problem. To reset your browser to default settings follow the steps below.,
Goto Control Panel--->Internet Options--->Advenced and at the bottom you will see reset button press it and confirm it.
Once done restart you system and give a try.
Best of luck