How can I find some Good Antivirus free??

I have my own Laptop in which I use Windows XP operating system. But unfortunately I have no Antivirus program to setup in my Laptop. So I am getting a Message that says that "Your PC might have at risk". Now how can I get rid of this message? And moreover I want to know how can I find a good antivirus for my PC. But there is a problem as I have no credit card and no Bank account of my own and I am a student so I have not enough cash for a Antivirus.
This is the main problem, so I can't figure it out how can I make my PC secure. I need a good antivirus which I can use for free. Can any one help me out of this problem? Please give some links where I can find some free antivirus which can be used for long term. Its very urgent. Please give me answer as quick as possible.
