Windu did not actually beat Sidious. It was just a part of the plan to turn Anakin to the Dark Side. Before Anakin entered he let himself be disarmed so he would appear like a victim.
I mean I know this was kinda "left to interpretation" so the kids can be content but anyone with a little bit of intellect or common sense can see that it was obviously staged.
Sidious lost the lightsaber duel to both Yoda and Windu. When the fight turned to spamming force powers, Sidious gained the upper hand.