Leopard keeps locking up :(

This started on me last night and just happened on me today, when I start the computer up everything is fine however after about a hour or so every program one by one locks up (all I get is that damn pin wheel when I move my mouse over). At this point If I try to force quit the app force quit box locks up! if I try to open activity monitor from the dock, the dock locks up. It just gets to the point where the entire system is no longer responsive and I have to hold down the power button in order to get the system to restart. I'm at a loss at what is doing this, id love any help with troubleshooting.