Anything by DU is terrible, evil crapware to be avoided! The nag ad you are seeing is nothing but an attempt to turn your money into theirs.
DO NOT use any task killers, RAM memory optimizers/boosters, battery savers/repairers, cache cleaners, antivirus, etc etc. None of that stuff is necessary. Such apps are actually counterproductive and some are just plain evil. They actually waste power and may disrupt critical system processes and degrade performance. Not to mention the nag ads many throw up as you have seen.
To understand why such apps are so bad go back in this thread and read posts # 3, 23, 27, 31, 33 & 180 - especially 180. That should be plenty of information.
http://androidforums.com/index.php?threads/896663/
Actually, it's a good idea to read the entire thread. Lotsa excellent information.
Android is excellent at handling running processes and memory with no help from 3rd-party apps or user assistance.
Just let Android do it's job as intended, relax and enjoy your phone.
Android since v1.0. Linux user since 2001.