I've got hold of a P4 1.8G 400/256 and when i install itsometimes it will power on fans run etc but not post and sometimes short burst of the fans then stop..looking at the cpu all pins are there and all straight, no signs of overheating/burning anywhere on the cpu. The mobo it is in is a GA-8SG800 with 768meg of pc2100
OK, the "spinny" fans mean there is power to the board.
#1 - Make sure the board is not grounded out. Check the mounting posts to make sure there are no "extras" touching the board. No grounds? Step 2
#2 - Default the BIOS. Leave the battery in and short the jumpers by moving the block over one pin for 5-10 seconds.
#3 - Remove all but one stick of ram and your video card. Make sure they are properly seated. Try again. No boot?
Read through the top post on "Motherboards & Chipsets" and try everything including popping the board out of the case again. I'm assuming you plugged in the 4 pin power lead into the socket next to your CPU and you have a decent power supply. This board should support PC-2100 and a 1.8G CPU.
"I am become death, the destroyer of worlds. Now, let's eat!