Is your computer part of a domain? If so, the computer will synchronize with the domain controller by default and the Internet Time tab will not be available.
If your computer is not on a domain, the Windows Time service may not be starting properly. Click on the Windows Start button, then click on Run. Type in services.msc and press Enter (or click OK). In the services list, look for "Windows Time" and make sure the status shows as Started, and the startup type is Automatic. If not, double-click on the "Windows Time" line and change the startup type to Automatic. Click the "Start" button and make sure the service starts properly, then click OK to close the window. The Internet Time tab should now be available in the Date and Time properties window.