How can you protect yourself against this and similar trojans?
My rule is to always, always obtain software from the company that makes it, not a third-party, and especially not an unknown third party or one who promises something for nothing.
There is no free lunch if the software originally cost money. If the software is really free, get it from the guy who wrote it, for free. But if the guy who wrote it charges, then getting it elsewhere "for free" is likely to cost you bigtime. Pony up and pay him what he charges, or don't use it and find something else.