This question is probably one of the most difficult ones out there. Where did you get your virus from? Well, if you have a virus pop-up that says what kind of virus it is, you can search on the Internet where it came from. However, it doesn’t mean you didn’t receive it from another place.
The most common way to receive a virus is via e-mail and downloads. You should be very careful when opening attachments from unknown senders. Don’t just get click-happy to get to your next e-mail. Your impatience could cause you a big headache if clicking on a malicious attachment. Also, when going to different websites, try not to click on 
any pop-up advertisements since they may be a virus or spyware. Further, try to stay away from the free download sites, such as Limewire. Those sites are nothing but trouble.
In the end, you will not know where you received your virus unless you re-trace your steps and think when your computer started acting slowly or strangely. Then, you might be able to put a finger on where you went wrong.