RAM Problem

[amadeodoom]

Hi Friends,
I have a problem with my RAM. I recently changed my RAm to 512 from 256. but after that when i restarted my computer it dose not show me the Full 512 RAM specifications, Instead it is showing a RAM of 386. so i wasworried if there are any viruses in my computer and i did a complete system scan with Symantec Antivirus. but could not get any virus. I am not able to understand what shall be the problem and why it is showing me less RAM in my computer. i am not being able to configure it. will there be any problem with my RAM slot or will it be any other problem. Can anybody help me out with this issue. Still after changing my RAM my computer goes slow.Can anybody help me telling what shall be the problem. i am running windows 2000 in my computer.

Thanks in advance to everybody.