You can browse the android source code using their GitWeb interface. If you want to check out a specific project (i.e. download the source), you will need to get git. For Windows there is a good native implementation available: msysgit. When you have git running, you can either clone a complete repository using git clone git://android.git.kernel.org/projectname.git
or just get the HEAD (the most current version of all files, useful if you only want to browse through the source) by doing git clone --depth 1 git://android.git.kernel.org/projectname.git
.
The project name is the top folder you select on the GitWeb page, for example platform/packages/apps/Calendar.git
for the Calendar app. Then the full command is git clone git://android.git.kernel.org/platform/packages/apps/Calendar.git
.