macbook boots up, let's me log in, but then I can't click anything or it restarts

This problem just started up 2 days ago, my computer died one night and the next morning when I booted it up, it wasn't working. It will start up fine and bring me to the log in screen, I can log in fine. My desktop loads up and all the files I have on my desktops are all still there, the dock loads up with all my apps (except for itunes , the icon is missing completely and there's just a blank space, when I hover over it it says iTunes). The menu bar usually shows up fine but sometimes its blank and sometimes it doesn't show up at all. Now here's the real problem: whenever I try to open an application, or even open up one of the files on my desktop, the files and dock all disappear for a second and then it all shows back up with an error message : finder quit unexpectedly. Sometimes this message doesn't even show up and everything loads back up, but the blue dot that indicates that an application is running is not next to Finder. so maybe this is a problem with finder? This is kind of an old computer and i use it all the time so I wouldn't be surprised if its finally just dying... The operating system is mountain lion but I'm not sure what number.. I can't look because I can't click on anything without it doing the restarting thing.