VAJIH102, whoever told you to replace the whole security system is trying to make extra money off of you. As gunshippilot mentioned earlier, you should shop around in the local market for a spare remote. Just make sure when you or the retailer programs the new remote, the receiver is reset and your other remote is also programmed at the same time.
Have you tried searching online for programming instructions for your specific model? It usually is a very simple and straight forward procedure, especially for after market units. If you can find the instructions online, all you have to do is buy the spare remote. Can you share the make and model of your security alarm? Someone here maybe able to provide you with better guidance if they knew more details.