anti virus

I have a computer whish has a virus, I tried to install anti-virus (Norton 9.5) but the machine didn?t allow me. The machine spybot has MacAfee antivirus on it ,also I tried to run it but it didn?t allow me. I went to safe mode but it didn?t allow me to run or install the anti virus. Any idea that can I get ride of it so I don?t need to format and install every thing all over again
Thank you