After you download the software, unzip the file to a directory on your harddrive. There should be a program called runInstaller.exe (or something like that). Double-clicking on the install program will start your software installation. By default, the software will be installed in C:Program Files, but you can install Oracle in any directory. At the end of the installation process, the installer program will fire up a utility called the Database Configuration Assistant (DBCA), which will create your first database for you. The DBCA wizard walks you through, step by step on creating your first database. When you get to the screen showing the datafiles, you can change the file locations if you do not like the directories the DBCA has selected for you. You will want to pre-make the directories for the database using Windows Explorer, no different than creating a directory for any software.
I highly recommend reading the Installation Guide documentation as well. This document should be available in the download. If not, you can view all documentation on Technet. This document contains specific instructions for your platform.
This was first published in July 2005