From your screenshot, the Basic Warez is a virus that has not been executed. You can download those.
However, the DDoS viruses further down, someone has already
Executed them, so they're running and belongs to someone.
The download center don't usually stock DDoS viruses though.
If you follow the puzzle servers, one of the early ones will stock one. However, people like to login and run it, so you check the server's software reset time, and download it before it gets executed.
Note: DDoS leaves a lot of footprint of your server. You will get attacked pretty quickly after DDoSing someone, so either pre-load an IP reset or just don't do it.