How to trace cause for unknown crashes?

I am using Ubuntu 9.04 . Many of the times I have faced a problem where the whole system just freezes. I can move the mouse freely across the screen but nothing else happens. There is no processor/disk access. I cant click on anything. I cant open up virtual terminals using ctrl+alt+2 etc. In short the only thing I can do is move around the mouse thereafter I have to manually reset the system. I was wondering If I can do something to find out what is causing this. Perhaps I could run some tracking/logging program by which I can at least know what is causing this?? or perhaps there are already logs in the system, if so how do i access them??
P.S I am a bit new to linux so bear with me if I am unfamiliar with system logs,commands,etc.