Hard Drive Filling Up With Files

Sounds like you have simply overloaded your drive. There are various ways to remove user files, the trick is to remove only those you no longer Need or want. I prefer to file old data off to and external drive and then delete the old way with finder the ones i copied. This can be slow but I have the time and I have greater control.

The os needs about 20% disk space open at all times to be able to work properly. Until you get there deleting may be a slow tedious task.

you will do yourself a tremendous favor by taking at least some of the load off you disk.

What size disk do you have? Updates can increase disk usage some as can ugrades. Is you iMac set to update and upgrade automatically? If you're not using it though, I assume it's turned off. Are you saying when you turn it on "suddenly" disk space decreases? Automatic everything might account for some of it. If you have a very small disk, additions will appear proportionally larger than for a very large disk.

Very unlikely you have a virus.

Then I suggest you get in touch with that site; I would never subject my Macs to a bittorrent site; a lot of them do include nice "extras" which may turn out to be malware. If the site advertises anything free that would cost money elsewhere, it is best to pay for the stuff from a legitimate site/developer.

Then you've had your answer all along.