Are you using a crossover cable? There are 2 different types of ethernet cables: straight-through and crossover. Straight-through is used for connecting to a router, whereas crossover is generally used for connecting 2 computers directly together, because it "crosses over" the wires internally so that the computer gets the data through the wires it's expecting them from.
To check what kind of a cable it is, look at the coloring of the wires. If the order of colors is the same on both, it's a straight-through, and if it's different, chances are it's a crossover.
Many newer ethernet cards automatically do the crossing over, so it doesn't matter what kind of a cable you use, which could explain the working connection between the PC and laptop.
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