Descarte was quite a radicle for his time, but you must understand that he saw himself as defending the christian faith. In my opinion, he did an unsatisfactory job of it, but at least he made the attempt to proove what he belives in.
His proof in god rests in two ideas, his defination of the "soul-substance" and in his defination of the imagination.
I cannot tell you much about Descarts defination of the soul-substance, but I can tell you the critique of it. Descarte said that the human body was sort of like a shell in which the human soul inhabitated, which is what seperated us form animals. The problem with his definition was that he said that a person insubstantial (no tangible) soul was plugged into their brains, which to him meant that humans could feel pain, and that animals could not. He used to string dogs to his fence and eviserate them to try and proove his point, a rather grusome practice in my opinion, one which did nothing to proove his point. Anyway, the problem is easily apparent, how does a totally non tangible, non-corpreal "soul" interact with the completely tangible, only of this earth "body". For that question he had no answer, becasue the only logical answer is that it cannot.
Secondly, his defense of God came from his definition of the human imagination. He said that humans could never come up with anyting new, theat they sort of "re-aranged the furniture within their mind" to create something "new". Becasue we can immagine God, (which I would argue we cannot), and we have never actually seen a god before, but still have the idea, then it must be becasue god planted the idea in our minds. The critique of this is that we can immagine things opposit's, and for example, by looking at our own imortality I can immagine something that could never die. I could, in threory, imagine an island in the pacific that has cities made of gold and is filled with beautiful women, but that does not mean it exists. I can imagine a 3 headed dog, even through I have never seen one, but does that means it exists? of course not.
As for moonlessnight's assetion of descarte's "perspective he needs God to exist" claim, this is probaply true. In his own words he described himself as the great defender of faith (the belief in gd) as he had his problems with the church. Keep in mind, that Descarte's plan form the begining was to come up with completely logical proof that God does exist, and that all his meditations were geared toward getting his reader in the right midset to except his proof's for god.