Windows will automatically release memory when programs ask for it. But if you want a nice little program which frees up memory (especially useful if another program doesn't release memory back to windows when it closes) try MaxMem which can be found at analogx.com.

One suggestion, MaxMem itself (like any program) takes up memory when it's running. So what I do is have a shortcut on the desktop and just run it when needed, but then close it after it does its memory freeing.