Malware is often disguised as, or embedded in, non-malicious files especially executable programs and software installation files. Nowadays, downloading freeware, even from well-known websites, often leads to malware infection because people are unaware of unexpected 'free' contents inside, see Figure 2. Personal data storage devices are also major source of malware spreading, apart from unsecured computer networks. Malware creators always focus their attack on bugs and week points in the systems. It will spread easily and causes wide radius of damage if computers in the network are running on the same version of Operating Systems.