"Were is the defrag located in windows xp home edition and is it good to do this to you hard drive if so how often???"

a. Should be in the Start, Programs, Accessories, System Tools menu. But since one can delete or move it, your system is your system. Nothing forces the shortcut to be there.
b. "good"? I will share that some seem to use their machine as nothing but a machine to run Defrag. I know someone that everytime I say "hi", they are defragging the drive. Myself? I have it run automatically once a month. Not an item I find to have a big payback.

"Does it speed up things?"

Some things.

"what about memory caching how do you clear it to make thing run faster?"

Given the fastest thing in your machine is RAM, why cripple this?

Bob