To fully understand POST codes you need a chart of what all the POST codes mean. So without further ado and without violating copyright, I will link you to the respective sites and you can view or download the PDF.
You can go here: AMI to see a PDF of the AMI POST and Beep codes.
You can go here: Phoenix to see a PDF of the Phoenix POST codes.
You can go here: Award to see a PDF of the Award POST codes
By default most POST codes start off as 00, FF or 88. After POST the codes will revert to 00,FF or 88. This is normal. Once the BIOS releases control of the motherboard to the operating system there are no more POST codes.
Below is a picture of a standard POST card. YOu can purchase one also if you follow the links.
Also known as a port 80 & 84 debug card. It outputs all the POST codes for AMI, Award and Phoenix BIOS's. It also comes with a manual which lists all the POST codes from each BIOS company.This is the required troubleshooting tool for anyone who works on PC's.Card has 4 LED's showing:( +5v,-12v,+12v, CLK ) (POST stands for Power On Self Test)
Read more about POST cards
So basically you need to find the POST code chart that matches your BIOS version. Once this is accomplished insert the POST card into a spare PCI slot and turn on the PC. Follow the POST codes. remeber that ,most of them are memory related. However not all. Slso kjeep in mind that if the POST card stops at certain code, say for instance '85' it may not mean that the problem is with whatever '2A' means. (Video, AGP on AMI BIOS) but it may mean that its the next code that is causing the problem. It may be stuck trying to progress to the next code. keep that in mind when troubleshooting with POST codes.
To learn more about the BIOS click here: Basic Input Output System.