Change startup folder in Outlook:
Step 1: Go to Tools and click on Options.

Step 2: Click on the Other tab and then click on the Advanced Options button.

Step 3: Under General Settings, you'll see the option "Startup in this folder" and it will either be set to Inbox or Outlook Today. Click the Browse button to change to another folder.

The next time you open Outlook, it will automatically default to showing the contents of the folder you selected in the last step. If not, you may be having an issue whereby the default Outlook icon created by Windows always defaults to the default folder, i.e. Inbox or Outlook Today.

If Windows is overriding your manually selected folder, you can remove the registry setting that forces it to the default folder. Simply navigate to the following key:

HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Microsoft Outlook\shell\open\command
You'll see the actual path to the Outlook.exe file along with any parameters.

"C:\PROGRA~1\MICROS~2\Office12\OUTLOOK.EXE" Outlook:Inbox /recycle"

Now all you have to do is remove the Outlook:Inbox portion in the value and that will prevent Outlook from automatically going to the default Inbox folder. It should look like something below. Remember that depending on your version of Office, the path to the Outlook.exe file may be different, so don't change that. Simply remove the Outlook:Inbox portion of the line.

That's all there is to it! Now you can startup Outlook with a different folder than the usual Inbox. Enjoy!