I'm not sure what is the exact frequency my self, but the switchmode powersupplies in car audio amplifiers commonly use around 32khz, PC powersupplies would use something similar I suspect.
As meltman said, the high voltage DC is "chopped up" by the high frequency fet's (fast switching transistor) by rapidly turning it on and off, like flicking a switch really fast, thus you get a rough AC current, I can't remember how the circuit attains the +,0,- cycle,(needed for the transformer to work) since the switched DC is initally just + and 0. (from what I recall)
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