I have 512 mb of memory installed but it only shows 412 mb ??

I have a HP 635n computer. I have 512 mb of ram installed. Two sticks of 256 each ddr 2100. When my computer boots up and on the system tab it only show 412 mb. I would like to know what happened to the other mb's. Is it like the hard drive and it gets damaged over time. Should I purchase new memory. I am asking because my computer seems to be slowing down a lot lately. Thanks in advance.

 

Mostly your video card using part of that memory.

 

Probably your video card,as Colemanguy suggests.

To check it, you could install Everest from MG at
http://www.majorgeeks.com/download4181.html
EVEREST Home Edition is a freeware system information, system diagnostics and benchmarking solution for home PC users.

See attached thumbnail to find details of your graphics card. Mine is from XP Pro and a Toshiba Tecra A7 duo core laptop, as an example. Bazza