There are a variety of ways that can be used to hinder the entry of new competitors into a market. There is tying (i.e., threatening that you will not sell a monopoly ingredient needed for the product if the manufacturer uses another vendor's non-monopoly ingredient as part of his product). There is de-commoditization by adding features to monopoly ingredients which insure that they will not interoperate with non-monopoly ingredients, such as the by-now famous error message in betas of Windows 3.1 when they were used with DR-DOS [DDJ, . There is decommoditization by inventing entirely new protocols, such as Token Ring, DECNet, or the MCA Bus, and making sure that your own products work best with these new protocols. Patent laws can be used to attack a competitor. Thus see, for example, http://research.microsoft.comwhich has the primary purpose of producing patentable technologies that can be used to deny entry to competition.
Perhaps the easiest, most common, and most misunderstood method, however, is FUD.