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This is my view of what Wormtongue's work was.
He was to feed Theoden some poison-like things that kept him weak and frail(I was rather skeptical about the poisonous drugs, but Tolkien confirmed this in one of his letters). Like any other old men feeling "old", Theoden would then not be willing to take care of all the small matters of his kingdom, being tired. Wormtongue knew this, and so approached the king with sweet words. Most people like words that praise you, however false it may be. Wormtongue probably did do some things of service, at first, to gain Theoden's trust.
He then used his men to keep all reports of happenings from reaching Theoden, or twisting them with his own little lies. He sent news of what was happening to Saruman. Then, he told Theoden that Eomer was plotting against Theodred for heirship(confirmed in Unfinished Tales). Theoden, not knowing any better and trusting Wormtongue too much, would have believed this. Grima would've kept a lot of Eomer's victories secret from Theoden, or belittled them while exaggerating his defeats. Gandalf then came and exposed Theoden to the outer air, making him realise that he was not as old and weak as Grima made him out to be. Wormtnogue did seem rather overprotective, didn't he? Gandalf helped Theoden realise that things were not as bad as they seemed under Wormtongue's lies. Notice how Theoden didn't really turn on Grima as suddenly as in the film. He still couldn't shake off his years of trust for Grima. So he gave Wormtongue a chance of aiding him, which Wormtongue denied. Even so, Theoden remarks that he misses Grima later on.
So, to answer your question, Theoden was manipulated by Wormtongue. Wormtongue was acting of his own free will, and as Gandalf points out, his reward was to be Eowyn. Saruman did not manipulate him, but rather promised him rewards. And Theoden was never under Saruman's "control", not in the books. There is no exorcism in the books, but rather wise words by Gandalf.
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