There are few more signs, for example - the bitcoin miners who utilise your gpu/cpu @ 100%, the processes are not even shown @ the Task manager.
What about the e-mail messages? It's not 100% that your friends will start receiving mails from you. It's more likely to start sending tons and tons of spam and still go undetected, until your ISP calls you about it.
And, the most-common ways to infect your pc - if the user is gullible enough to click on this or that link sent to them over instant messengers, facebook or e-mail.