I have an HTC EVO but its a love hate relationship with my phone. I love what it can do but it sucks on the battery.
After testing a few antivirus programs that you can download onto your EVO I came to the conclusion that unless you have your phone plugged into your outlet, an antivirus software program will drain your battery life within an hour.
Good news is that the HTC platform has not experienced any viruses yet unless your downloading applications that have not been verified.
My best bet is only download aps that are from trusted companies.