RE: How do you know your computer has been hacked?

Just sharing a few more ideas:

You notice strange emails in your inbox with error warnings, like "server cannot be found." This may mean that someone has hacked your email account password, and they're using it to hack your ISP's email server. Most likely, the hacker has turned your email account into something like a bot, sending out numerous spam emails to others. Because their victim email list may be outdated, some of the addresses may have been closed. Consequently, when the hacker-spammer uses your email account to send spam to the bad addresses, the affected ISP servers will send you a response indicating "no such account on this server." It can be puzzling in the beginning, because you never sent the original emails.
You receive an email or phone call from a job recruiter or corporate HR claiming that they've already been talking with you via emails. This is easy to miss, especially if you are actively looking for a job. But there are some unscrupulous recruiters who will create a fake Google Gmail account in your name, and post a fake resume (they patch together your job history from your public profiles on LinkedIn, Monster, Dice, and other sites). Your resume will contain the fake Google GMail address as well as a fake phone number (a Google VoIP account). The hacker-recruiter then starts looking for jobs for you, sending out emails in your name from the fake Gmail account. It sounds great so far (free labor), but, unknown to you, they respond to all inquiries insisting that you will only accept C2C opportunities so that "your recruiter" gets a cut. If the hacker-recruiter finds you a job, your employer will pay an extra fee to this guy that you will never know about (and may have been the reason why your original starting salary was reduced). Even worse, though, the hacker-recruiter will be turning down a very large number of possible interviews because those employers did not want to pay the C2C referral fee. In the long run, the hacker-recruiter stole your identity, engaged in contract negotiations without your consent, and turned down interviews that you would have liked to pursue.