The version you get free with Windows is Outlook Express and the version in Office is Outlook.
They are slightly different. The one in Office has a calendering system which Express does not.
But Express has a newsgroup viewer which Office does not.
I dont think they share files or directories so I think you CAN have both on your PC.
When you come to install Office you can choose NOT to install Outlook. Dont take the defaults but select the Custom option (I think it is called).
Here you can choose which parts of Office you do and do-not want to install.
So choose NOT to install Outlook if you wish.
Here is an article you may find useful which compares them.
http://office.microsoft.com/en-gb/help/HA01056 5781033.aspx