RAM is used dynamically by Windows - so what's left varies depending on how much you have. Windows tries to use it in the most efficient manner, and will use more RAM if more is available, but never all of it. It also uses hard disk space as *virtual ram* to make up the extra it really needs.
So - I appreciate you might like to know, but a little more information on what you have, and why you have asked, would make for a more informative answer.
I have an old freeware program that displays Day/Month/Date/Time/Amount of Memory (RAM) Left, in the Title bar. You're welcome to it if you want to email me, it's only about 60KB. I can no longer find a link to it to post. I have just tried it again and it works okay in XP.