Trying to detect cheating programs on the client is, in the end, quite hard, because the cheater controls the system the cheat is running on. Various cheat systems will map the memory of the game process, and patch it in-place to install the cheats, so there's no trace of any injected DLL. And, because the process is patched, it can't successfully check for other files or programs or processes or DLLs on the system, because those calls can be intercepted by the patch.
The extreme end of this is using virtual machines, or even hardware loopback. It's pretty common to build robot arms and cameras the read the screen of a Tetris game and plays it -- nothing is amiss from the point of view of the computer the game is running on!
A more robust way to detect cheaters is to use a multitude of signals -- social signals (reports,) outcome signals (hit/miss ratios,) response to slight changes in algorithms, and so forth. Then, use matchmaking to make people who play the same, play together. If this ends up putting cheaters in a group of players for themselves, that's great! If it means that cheaters and "pro" gamers end up playing against each other, then that's still not so bad, because the cheaters will fail hard once they get invited to a tournament :-)
