I am generally what is called on the USENET newsgroup alt.atheism a 'weak atheist'. That is to say, lacking any convincing evidence, I don't believe in God(s). In general, I take the position of not(believe(God)) - not actively believing there is a God. Obviously I haven't investigated every religion in detail, but lacking any convincing evidence, I return the Scottish verdict of "not proven".
Note: this is not the same as "agnostic". As formulated by the inventor of the term, Thomas Henry Huxley, it means an active belief that the issue is undecidable or unknowable. I simply regard it as unknown, not unknowable. I suppose I might be called a "non-gnostic". Details here.
On the other hand, of the religions I have investigated, I have specific reasons for rejecting them. In particular, for the traditional monotheistic religions (Judeo/Christian/Islamic conceptions of 'God'), I believe them to be internally inconsistent and illogical. In these specific cases, I take the position of believe(not(God)) - I actively believe a God of those types does not exist.
When I say I'm an atheist, there are several questions that get asked over and over again. (How politely they're asked depends on how pugnacious the questioner is.) I've written down my answers to those questions so I don't have to go into tedious detail, I can just point people here. Your question is probably answered below: