What was the first chess engine that could beat the world chess champion when running on a standard desktop playing at standard speeds (i.e. not blitz chess)? For concreteness, say a $1000 PC.
In a November 2006 match between Deep Fritz and world chess champion Vladimir Kramnik, the program ran on a computer system containing a dual-core Intel Xeon 5160 CPU, capable of evaluating only 8 million positions per second, but searching to an average depth of 17 to 18 plies in the middlegame thanks to heuristics; it won 4–2.
A standard desktop today is significantly more powerful than whatever machine Deep Blue was running on in the mid-1990s against Kasparov. Since Deep Blue was the first engine to beat a world champion, that's the answer to your question.
Note that there may have been an engine before Deep Blue that, if it ran on a modern day desktop, could have beat Kasparov. But we never saw such a match happen so it's just speculation to say any earlier engine than Deep Blue.