Does anyone know of a good anti virus software thats free?

problem is is i have a non genuine windows through no fault of my own i only found out after i got the pc but 1 of my friends has managed to get me a ligit 1 but im unable to install because a virus has found its way into the start up menu which is also now affecting my desktop which keeps dissapearing after i log in if any1 can give me some advice on what to do id be very gratefull i would buy an anti virus software but i am abit low on money

Does anyone know of a good anti virus software thats free?spyware removal

If you want a free anti-virus I recommend in order of popularity, but I leave the decision to you (install only one):

1. AVG Free

a. A simple interface

b. Can detect about 82.82% of viruses

Cons: low virus detection

2. Avast!

a. Real-time protection

b. It has an option to delete a virus once found

c. Can detect about 87.46% of viruses.

Cons: Too many icons and confusing user interface

3. Avira AntiVir

a. It can detect malware better than Avast and AVG

b. It's cute umbrella icon

c. Can detect about 94.26% of viruses

Cons: A very sluggish update

Does anyone know of a good anti virus software thats free?virus protection software