Have you made the necessary modifications to IIS? These are well documented in the PIA NTP, or you can run the Visual basic script that comes on the TM 3.1 CD called "Configure2003Server.vbs".
If you have run this script, or have correctly made the IIS modifications manually, then you need to make sure that you've complied with the requirements that are delineated in the NTP in a section called Local Security Policy. Essentially, you must make sure that the Users group has certain user rights. If you failed to make these changes prior to installing TM, you need to uninstall TM, restart, make the requred LSP changes, re-install TM, restart, re-PEP. At that point, you should be able to login to the web interface.