Windows designate a part of the operating system to store information about all the hardware and software installed. The windows registry is similar to checking into a hotel. You sign your name on the books or fill out a guess card, although now your information is entered by computer. When you check our (uninstall the program or remove the hardware) that information is still in the hotel files (or in the registry of the computer) The registry has what is know as keys and sub-keys. Keys and sub-keys are coding (binary) which tell windows what type of program you have installed or hardware and if the software has a limited time to operate before shutting down. Thus, trial programs no longer work after a certain time and if you uninstall the program and re-install, it still will not work, because information about that program is still in the registry.