There are two main problems with connecting Outloook and Exchange over HTTP when you have Sharepoint installed on the same server under the same Web site.

1) When you try to use Microsoft Outlook Web Access to browse the Microsoft Exchange Server 2003 client, you may receive the following error message:
Page not found
This problem may occur after you install Microsoft Windows SharePoint Services on a server that is running Microsoft Exchange Server. The Windows SharePoint Services ISAPI filter handles all incoming URLs. When you browse one of the Exchange Server 2003 virtual directories, the ISAPI filter does not locate the URL path of the folder.
Solve this by following the instuctions to exclude directories here: http://support.microsoft.com/kb/823265 but there is an extra directory you need to exclude that is not listed. Make sure you exclude the “rpc” directory.
After solving the first problem and you are still having issues, move on to the next problem:
2) Problems occur when a user installs Microsoft Windows SharePoint Services 2.0 on a computer running both Exchange Server 2003 and Microsoft Windows Server 2003. The deployment causes Kerberos authentication to be disabled in Internet Information Services (IIS) and can result in incorrect handling of Outlook Web Access and HTTP over RPC requests to a server running Exchange.