Malware Detection and Removal.  It's hard to tell at a glance if a computer has a virus. Only with a good updated antivirus you can know that by sure. However, there are certain symptoms that indicate the possible presence of virus (could be virus unrelated too): Unusual slowness, without any apparent cause, may be caused by several reasons: too many applications working at same time or network problems, but also to a malware infection. One reason you can not open certain files or programs is that has  been eliminated by a virus, or you've deleted the files they needs to function. One of the most common effects of virus is missing files and folders. Ads appearing on screen or unexpected text messages may be a clear symptom of virus infection. Generally, these notices contain unusual text (absurd, humorous, offensive, aggressive, etc). The computer turns off suddenly for no apparent reason and restarts or crashes. Some viruses need that happening to wake up and ensure its operation, so they cause such situations. If a program closes suddenly while we are working with it, We have reason to suspect an infection. 1Energy1