I can't enable Windows Authentication because the option doesnt exist under Authentication in IIS7.

When configuring Instant Archive Viewer to use Windows Authentication in IIS 7 on a Windows 2008 Server, by default there is no option to choose Windows Authentication. In this case you will need to add the role in the Server Manager.
To configure Windows Authentication for IIS 7.

1. In Server Manager, under the Roles node, select the Web Server (IIS) role.
   
2. In the Role Services section, click Add Role Services.


3. The Add Role Services wizard appears.


4. Under the Security role service, select the Windows Authentication check box.

5. Click Next and verify the role service changes to be applied.
   
6. Click Install to start the IIS and ASP.NET installation process.
7. Click Close to close the Add Role Services wizard.