Some viruses can be harmless while others can cause major problems with computer data. Basically, a computer virus is a threat to the security and integrity of your computer system. There are several ways one can get a computer virus. Be alert of any changes. Executable files may be larger than their usual size.
Computer takes longer to start.
Less available memory. Programs take longer to start. Unexplained missing files. Network unusually busy. Unexplained graphic images, sound, videos displayed on the screen.