Well, technically you can never be 100% sure, that your system doesn't have a virus.
That thing, which happened to you, may be either a) hacker attack (with low probability) b) scheduled task of some bad background application (more likely).
So I'd suggest to try this utilite:
After installing an opening it, go to the menu Tools -> Startup , check tabs "Windows" and "Sheduled tasks" and look for suspicious entries. I usually pay the most attention to the "Publisher" and "File path" aspects of applications, listed on these tabs.
This is a good idea in such a situations, too
Too bad (for us) that developers of malware become more and more sophisticated with time, and constantly learn to hide their malware better. The worst case is when they infect legit applications, adding their code to them. In this case, you will likely not be able to distinct activity of legit application from activity of malware through checking task manager or list of startup applications.