First, we know nothing about your system.
What is your OS, how much RAM do you have, what is your CPU.
Why do you feel you need to free up memory.
Second, for the most part you shouldn't need to free up memory, if you have XP that is relatively good at managing memory.
Third, the best way to optimise memory is don't run non-essential applications on boot, many applications on installation have themselves run on boot when they aren't really essential, media players are a common one and you only need it when you click a media file.
Forth, when you have done with a program shut it down if it isn't one you use regularly throughout the day, browser, email, etc.
Fifth, RAM is relatively cheap and easy to install so if you have memory issues that you feel you need to free it up you may well be better upgrading the existing RAM.