If your are using Outlook 2007 with an Exchange 2007 server, then AutoDiscovery may have been configured. When you login the first time you will see a dialog asking if you want to allow server settings to be configured automatically. The dialog has an option to not be shown again.
If you first select to deny autoconfiguration and to not show the dialog again, then later want to reverse the decision, you will find you are stuck. At least I have not been able to find a "nice" way to enable the dialog again.
However, I have found the registry value that controls the dialog. If you delete that value, the dialog will appear again so that you can change your decision. The registry value is:
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\AutoDiscover] "SkipRedirDlg"=-
You can delete SkipRedirDlg using the Registry Editor (RegEdit.exe). Or, you can put the lines above in a registry .reg file and double-click it. Below is a file which does just that.