I apologize for the late reply. No one is intentionally over looked here. Did you get your issue sorted? If not, please provide which Operating System you have on your computer and I'd be happy to help you.
You ask how this computer got infected when you don't use it for downloading. There are several ways for a computer to get infected. Downloading is not the only source for infection to be invited onto a computer. I have known computers to get infected that have never been used to connect to the internet.
Here are a few ways infections occur:
Poor surfing habits (visiting bad sites)
Connected to a home network or if your work computer is part of a larger network (don't even need to connect to the internet if sharing files)
From Mobile Devices
Not keeping all software up to date
Transferring files with USB Flash drives
Please read the following links as well:How did I get infected in the first place?
by Tony KleinHow Malware Spreads - How did I get infected
by quietman7How to prevent Malware: