Does not work for people using restricted drivers!!!
Today, most computers have more than 3GB of memory. But, in ubuntu there are 2 specific versions for those who use under 3GB of memory and for those who use more than 3GB of memory (32 bit and 64 bit). So for those who use more than 3GB of memory (64bit ubuntu) , it is a bit of a hassle to install to install most applications which are compiled for 32 bit. Wndows Vista and Mac OSX have a PAE kernel built in so this is not a problem for them. But, unfortunately, Ubuntu doesnt have a PAE kernel by default. For those who want to use more than 3GB of memory, there are 2 options:
Option 1:
Use a 64 bit version of Ubuntu
Option 2:If you are using a 32 bit version of ubuntu and not keen to switch to a 64 bit version of ubuntu, the run the following command in a teminal.
sudo apt-get update
Then run these:
sudo apt-get install linux-headers-server linux-image-server linux-server
sudo reboot