Computer viruses are pieces of code that can copy themselves and spread from one computer to another. There are several types of viruses including boot viruses, program viruses, multipartite viruses, stealth viruses, polymorphic viruses, macro viruses, and ActiveX viruses. Signs of a virus infection include applications not working properly, files changing size or date accessed for no reason, and systems slowing down or crashing. It is important to use antivirus software like Norton, McAfee, or Windows Defender to detect and remove viruses to prevent data loss and system damage.