Maintaining a PC Without Anti-Virus Application

My uncle knows how I love to have my own computer, which I can use for working online. So last month, he gave me one desktop computer. It's kind a old model but since I can't afford to buy a new one, I gladly accepted it.
But I found out that there's no anti-virus application installed on the unit.
My friend suggested downloading free anti-virus software online, but I'm not sure if that's a good idea.
So, I want to consult to the experts. Is it really safe to download and use the free anti-virus applications online?
If it's not, how can I use my computer for my online job and keep it free from viruses?
Hope you could help me.
Thanks a lot.
