details of windows system files

Where can I find a resource that explains what each file in the Windows directory does? My Windows folder has grown to 850MB and I know that most of whats in there is garbage. I am also looking for a resource detailing the registry.

[DR]

>You can right click a folder and select properties for quite a bit of information on a file.
>Check the TEMP folder in Windows. It is supposed to empty when you shut down but often leaves files that can be deleted.
>Is temporary internet folder filling? It has to be deleted from Control Panel/ Internet Options.
>Regedit.com is a good site to learn some about the registry. Be ready to rescue yourself if you start learning by doing.
>I personally use Cleansweep 4 to moniter what I load. It also cleans the internet files and has a registry tool.

[DR]

>Also check the recycle bin. I believe it is usually set to 20% of hard drive space. This can take up a lot of space. You can reduce the size by right clicking the icon and choosing properties.
>You can also empty it if you are sure you won't need anything that has been deleted.