Windows XP Pro, usually after a while of me leaving it on, when i have a few progs running and minimize them it takes a while for them to come back up when i select them again(about 10 secs). i usually have about 4 progs running at one time with the internet on all the time.
P4's have a thermal cutout, when the CPU reaches a certain temp it stops working at full capacity to prevent further overheating and runs at something like 800Mhz I believe.
This would seem very slow compared to it's rated 2.4Ghz.
I think you may have a heat problem, you need to take the side cover off and check for dust in and around the heatsink also check that the CPU fan is spinning freely and not obstructed. You can buy cans of compressed air from computer retailers for blowing out the dust for about 6 quid but be careful not to dislodge any jumpers on the motherboard.
Also check the case fans are not obstructed and are spinning freely.