Complex sentence: The worm also made some systems unstable by its workings, and once the patches had been installed it rebooted the system, because of these this was still seen as a threat, and a patch was released by all major anti-viral companies.
Split into simpler sentences: The worm also made some systems unstable by its workings, and, once the patches had been installed, it rebooted the system. Because of these effects, the worm was perceived as a threat, and a patch released by all major anti-viral companies.