System Slowing down

My Xp system is slowing down on boot, function, and shutdown. I have plenty of hard drive available. I have tried cache clearing, disk cleanup, defrag, ad-aware, and anti-v scan and have not seen anything that looks like a problem in the results (butI don't know much!). It finally reached the point today where the system couldn't boot up. The desktop background would appear, but it would never reach the point where desktop icons would appear or any programs became functional because I would only get the hourglass. I had to push the power button about 4 times before the system would boot. What might be causing this and how can I prevent it? Thanks.