For one is that a stock heat sink/fan combination being used or did you install a 3rd party cpu cooler? With a stock hsf you may be seeing the thermal pad giving out if the fan isn't covered with dust and debris. Or it may have simply given out on you. From your sig the last two things are those to consider.
With a stock hsf you can often replace thermal pad along with using some Arctic Silver 5 or even the new Liquid Pro which costs a bit more. The common choice made by most is to toss the stock and go for a better 3rd party cooler.
Well first thing to do is to make sure that the fan is spinning when you turn your pc on. The Cpu fan may not be connected to the mother board. I've seen a couple of people were this has been the problem. Also give your case a good clean out with some compressed air, get rid of any dust that may be clogging up your system causing the air to not be circulating as well as it should.
That won't matter! The temps inside the case will. The lack of good air flow especially with a P4 model cpu and stock hsf will see high temps. You waited until the thermal compound was dried before first running the system? When first applying Artic Silver or another compound you let the system run for a few hours like 3 to 4hrs. to heat up the compound and then shutdown for several after where the bonding process starts then. It usually takes about two weeks to see a complete biond afterwards. You will probably now have to clean and reapply a fresh application and follow these steps.