Charles Conway writes that after installing the SAM Intercept 4.5.3 update (and the 03SAM 98 virus defs file), he would get a system freeze after selecting Restart (from the Finder's Special menu) on his PowerBook 1400. If he turns Intercept off, the problem goes away. This did not happen on any desktop Macs. Having SAM Intercept load later in the startup order may work-around the problem. Others have reported this problem as well, as noted in a thread on the Symantec Tech website
Update: Jon Bergh claims this can be fixed simply by giving the SAM application more memory.