How a computer virus infect your system and how does it spread ?

I think a virus infect file by file.. yes first is goes in our windows directory.. and corrupt the main folders and files one by one.. some virus are very very dangerous.. and also infected our whole windows..

there are many types of viruses. The way they spread are not similar to some kinds of virus. There are worms, trojans....etc...well, a virus can affect your entire computer or your system administration on your computer. Better use an application to freeze your system admin to protect it from viruses..deep freeze application for your c drive.

I think virus spread through the internet and the infected disks. I also had computer infections couple of times and had to format the hard disk. Sometimes it corrupts the data and also sometimes slow down the the computer. Some sites are also blocked when the virus enter the computer.

I remember when I had a virus or a Trojan and my computer keys started to act up. I also had the computer slowing down, as if someone was trying to see what you were doing. I think there were weird programs put on it with weird symbols on top. I tried to get rid of it, but the process was so complicated that I had to reformat my hard drive. It has not effected this one, because I have a good free antivirus (Avira) from Germany. And that warns me if there are any Trojans or Viruses coming up.