Could you be a bit more specific on what kind of guide you are looking for? Do you mean a guide on how to build a computer or a guide on what the components actually do?
Wikipedia is a very good resource for learning about computer hardware and how it works. For example, here are some articles regarding CPUs, memory, and GPUs.
http://en.wikipedia.org/wiki/Central_processing_unit
http://en.wikipedia.org/wiki/Random-access_memory
http://en.wikipedia.org/wiki/Graphics_processing_unit
Hanging around these forums is also a great way to pick up information.
If you are looking for a guide to build a computer, here is a very good video:
http://www.youtube.com/watch?v=W4Js2A1qdB8