That means XP is no longer secure enough to use on the UC Davis network—or to use online, period. If you still use XP, you need to move to a newer operating system.
UC Davis policy prohibits use of unsupported OS or application software on machines that connect to the campus network. Unmaintained software is a common entry point for viruses and malware, which can then spread to other UC Davis computers.
See the UC Davis Windows XP Security page for more information.