Integrated graphics would be my first guess.
I have a Gateway MT6154 laptop with Windows XP Pro. I've upgraded the RAM from 2 GB to 4 GB. The bios and CPU-Z reports there is 4 GB installed, but the system properties reports 2.62 GB.
System Properties: http://i44.tinypic.com/2vl4lmc.jpg
I know that XP has the 4 GB limitation, but why is it reporting 2.62?