A couple things here:
1. You seem to be trying to download MTA:SA, but this is the support board for MTA 0.5r2.
2. If trying to download MTA 0.5r2, some Anti-Virus programs think MTA 0.5r2 is a trojan virus. This is a known false-positive caused by many things in the way MTA 0.5r2 works and is packaged, which makes the Anti-Virus program flag it because of it's code's behavior. It does not actually contain any malicious code whatsoever.
3. Do you have any Norton products installed? If not, then you have a Browser Hijacker, which is a form of Spyware that hijacks your browser and redirects it to certain pages for advertising, porn, or malicious purposes.
4. If you do have any Norton products installed, and are they legit copies? Is Norton itself saying anything too? Is the URL real and correct?
5. Due to issues with both MTA File Archives having issues right now, please use only the links from this topic if trying to download MTA 0.5r2: https://forum.multitheftauto.com/viewtopic.php?f=136&t=33529
6. If trying to download MTA:SA, only use the links found on the Wiki or Main Website Page.
7. If seeking support for MTA:SA, please use this board here: https://forum.multitheftauto.com/viewforum.php?f=103