Unused RAM in Windows XP (and previous Windows OS versions to Windows Vista) is the result of poor and senseless approach to prefetching, hence why Microsoft introduced Vista SuperFetch in Vista - a sensible approach to memory management and prefetching.
In essence, if you find yourself needing to use a RAM optimizing / defragging program, then you've got memory management problems on your system which you should be dealing with first / instead (for example, either due to memory-leaking processes, too many processes or not enough RAM).
Using an analogy, running a RAM optimizing / defragging program to 'clear your RAM' is like pumping water out from your flooded basement, without fixing the cracked water pipe. Fix the cracked water pipe and you won't need to keep pumping the water out of the basement. Fix the cause and you shouldn't have to use a RAM optimizer.