I have a problem with Office XP Professional SP3. This problem is happening more and more frequently. Here is the Lab environment:

Vista SP2 computer
Students logon to a domain environment
Students save/open there work on a Network drive
Computer has Office XP SP3 installed
Symantec Endpoint installed
Lab environment with about 30 Vista computers

The problem is this:

A student logs on with there name, they go to the Network drive on the File server to open there PPT document. Within minutes of opening the PPT document, Powerpoint will go white and say NOT RESPONDING. This problem happens on about 10-15 computers but not all 30 computers. This problem will happen repeatedly on the same computers.

What is causing this problem to occur? How do I fix this problem? On the worst computers, I have performed a Spyware scan but didn't reveal anything unusual. Could it be a Windows Update?