There are several techniques being used by griefers to crash servers and cause individual users video cards to blow up. In both cases they seem to be using a scripting language (LSL), not some sophisticated virus to cause the attacks.
A computer operating system, as used on SL servers, knows the applications, processes and services running at all times. During normal operations, it is likely that any program exceeding normal limits could be identified immediately. So, it is in the realm of possibility that countermeasure software could identify programs causing demands that can crash a server.
The same is true of the operating systems running individual client SL viewer software. Perhaps, if the server is sending the types of graphics overloading used to blow up an individual video card, it wouldn’t be necessary to have client based countermeasures.
Every server and client on the Internet is already running anti-virus software. This is similar in concept except we know already that all the attacks will be generated by LSL scripting language, so it is relatively simple to identify the internal processes that are being overtaxing the server resources. Perhaps countermeasure software would place a performance burden on client and/or server software, however, a basic protection could be built into the SL server software. This in the realm of possibility.