The Stanford Network Registration Tool (SNRT) checks a few important settings on your computer before it will allow it to connect to the Stanford networks.
One thing the tool checks is OS updates. If you are missing a large number of patches/updates for Windows, SNRT can download and install them, however you may want to consider going directly to Microsoft Update to download and install the necessary updates. Usually this involves opening up the program "Windows Update," which can be accessed through the control panel. This process may take a while and may require you to restart your computer a few times, return to the Microsoft Update site and check for remaining updates.
Once you have done this, you can then run the SNRT to be approved for registration. Installing BigFix is not required (only recommended) to pass the check (if you're fully patched you can click the *Continue* button in the lower right).