Lets say you have a server whith 8 GB RAM installed and when you are getting ready to put the server on production you discovered that it is only reporting 4GB RAM. Don’t be alarmed! It is not a hardware fail or misconfiguration error.
This is one of those “I don’t know why Microsoft” does not set as default on their OS installations, anyway.
The only thing you need to do is go to c:\ and look for the boot.ini file, open it using the always trustable Notepad and add /PAE to OS boot command line:
\WINNT="Windows ???? Datacenter Server" /PAE /basevideo /sos
Save the file and then gues what? Yes you do! reboot your server and enjoy of the power of 4 GB of additional RAM.
More information here.