If you have been seeing these lines for over a month then it would appear to have been happening when you had the video card driver
problems just three weeks ago. See the thread here: Emachine Netbook 350 Video Controller
It would appear that the previous problem was not solved then if you could see the black lines when you installed the new driver.
Were there black lines before the new OS was installed?
As you mentioned viruses, have you got an up to date anti virus software installed?