Is there a temp sensor underneath your CPU socket on the motherboard? You can use that in Speedfan to get an approximate temperature reading.
The reason that your fan is running so fast is probably the CPU temp. At 3.8 GHz, it's probably pretty hot. What brand of cooler is it? If it's a stock P4 cooler, you might want to take it back to 3.0 until you get something better - if you want the P4 to last, that is.