Open Task Manager > Processes tab > click on "Memory". There you will see which processes eat your RAM - ordered by size. Maybe you have too many startup programs. Have a look in msconfig and disable those that you do not need (which are probably all). On the other hand, there is nothing wrong with the system using all the RAM. I assume it's all paid for - LOL.
But when you have excessive usage (like the 99%), you can also kill the culprit process in task manager. No need to shut off the whole system.