Mac OS X 10.5.8 keeps saying application not responding

Hi I have a macbook pro (2009) running on Mac OS X 10.5.8 and I keep getting 'application not responding' on every application I have opened on my laptop usually after not usuing them for a few minutes - when I go back to using the application (doesn't matter which one it happens to all of them Safari, itunes, icalendar everything) I can't use it as it says application not responding so I have to QUIT the application and restart it to use it. Which ***** as it is a pain and I tend to lose things now if I get away from that application for a second or walk away from the computer - what is wrong help! I am scared one of these days it won't restart and I will lose everythingI