Trojan.Agent/Gen-Multi is a heuristic detection for several anti-virus solutions. If your anti-virus detected the Trojan.Agent/Gen-Multi virus it is possible your computer is infected with more malware. Malware is an umbrella term that refers to any software program deliberately created to perform an unauthorized and often harmful action.
Since the Trojan.Agent/Gen-Multi is a general detection it is possible you picked it up at a compromised (hacked) website or by downloading a malicious file. Nevertheless, you need to check your computer, it is possible there is malware left that is not detected. Malware programmers tend to use a dozen of methods to prevent detection by any antivirus solution.