This is how it could happen
Anakin and Obi Wan go to rescue Palpitine oly to realise Dooku is there awaiting them with his sabre switched on
Dooku: "I thought you two would have stayed away from me after our last confrontation. Especially you boy."
Anakin: "Don't be so head strong old man. I have learnt alot since then."
Obi Wan: "Anakin, keep your concentration. Use the Force."
Dooku makes the first attack. He fights both Anakin and Obi simultaniously.
Dooku disables Obi Wan and goes for Anakin.
Dooku: "Your powers are no match young padawan. You couldn't defeat me before, and you can't do it now."
Anakin: "Don't be too sure. I am a Jedi Knight."
Dooku: "And I am the Dark Lord of the Sith."
Dooku sends a powerful lightning shock to Anakin. Anakin just stands there and absorbs the shocks.
Dooku is suprised and enlightened.
Dooku: "Obi Wan has tought you well."
Anakin: "He has tought me well enough to defeat you."
Anakin and Dooku battle it out. Anakin with a swift moove takes of Dooku's hands and has his sabre in his own hand.
Anakin: "You are now going to be sentanced to your crimes Dooku."
Dooku: "That's Sifo Dias to you."
Anakin takes of Dooku's head and helps Palpitine out of the chair...
A bit corny I know but I rushed it in five minutes.