Advanced Task killer is a definite Do Not Install! It's unnecessary and counterproductive! Don't use any task killers, RAM memory optimizers/boosters, battery savers/repairers, cache cleaners, etc. They are counterproductive, waste power, disrupt system processes and degrade performance.
Manually futzing around closing apps, killing apps, installing RAM boosters, etc is simply incorrect operation of Android. Among other problems these apps cause, it means you're not taking advantage of Android's optimization capabilities, resulting in less than optimal performance and reduced battery life. Android can learn from your usage habits and optimize itself to suit. It also learns when lag occurs or other system glitches and adjusts to cope with them better. it runs better and better as you use it. But it has to be allowed the chance to do so. If an app intervenes and kills processes, the learning/optimization process is broken. The intervening app essentially causes incorrect information to be sent to Android, degrading performance.
The developers of Advanced task Killer and many, many other apps clearly show a fundamental lack of knowledge of, or disregard for, basic principles of Android operation.
This link should help you understand. It's a good explanation of how Android handles multitasking, processes, RAM, etc automatically for written by one of the authors of Android:
Android Developers Blog: Multitasking the Android Way