They have installed 512MB

They are several ways to check your physical memory.
1. When your computer starts, the system will check the physical memory, and the number will be displayed on the screen, but they will mot show your 512MB, instead they will show you 524288KB, 1K=1024bytes.
2. When your computer starts, keep on pressing DEL key, it will bring you to CMOS setting. In most case you will see your physical memory number there.

I have a computer which has a shared memory with Video. The physical memory is 128MB. When I checked my Computer's properties, it shows only 112MB. The rest of 16MB memory are reserved to Video.

In CMOS setting you can change Showed Memory Size, if you really do not like to see 480MB, try to reduce your Shared Memory Size to 4MB, so you will see at least 508MB, it will hurt some programs to run smoothly.