Can I overload my netbook if I use a charger with a higher wattage than the one indicated for my netbook. I know that the charger has a protection in case of an overload. But what if, for example, the wattage rises and the protection stops at 65 watt. But my netbook can only take 25?
No, Wattage is not a issue. Wattage (power) is determined by the voltage and the amperage (think "pressure" and "flow"). If you have the correct voltage, then normally every laptop will be pulling less than the maximum wattage from the the power supply.
If a laptop or any device tries to pull more amperage than the rated power supply can deliver, then bad things happen: the power supply will overheat, fuses might blow, the voltage will go below the rated amount.
However, the opposite case of a laptop or a device pulling less amperage (and thus less watts) is a good thing.