I have Netbook running Ubuntu 13.10 32 bit with 1GB RAM installed, but when I go to "About This Computer" shows only 992,4 MiB?
That is completely normal. Part of the RAM is reserved to the system to use it so it doesn't show as available. In my case I've got 4 GB and I get 3.8 GB under "About this Computer".
Run "free" in a terminal and it will tell you how much memory you have got, and how much it is reserved. Just type in a terminal:
If you want to be sure it's not a case of corrupted RAM, check the amount detected in your BIOS when you turn on the computer. Also, you could run memtest on the GRUB menu to check that all the RAM is working.