I've been having this problem too. Sometimes it happens straight from bootup and the gadget I have that shows processor/RAM use doesn't even come up. I think it's been what's kept my login screen from coming up somtimes too. it just hangs at a black screen.
Sometimes it happens within 30 min or so after boot-up. When I've managed to get a task manager up, there's nothing to indicate a process that's maxing out the system. It might say '100%' at the bottom, but adding up stuff in the CPU column doesn't add up to that, even making sure to show all the processes, not just mine.
Most recently after booting up my Norton was turned off or thought it had expired or something. I downloaded AVG and got it to run a full scan and all it found were cookies.