Unplug AC power from the PC...open it up and reseat the RAM.
Leave it open, plug it back in and run memtest86+ for a couple of hours.
Let us know what happens.
Recently my computer has only been recognizing 512mb of ram on startup, while I know in system (control panel) it recognizes 2gb. I am a bit confused as to how this could happen, but I notice the computer is significantly slower in all aspects. I've run avsst antivirus, but nothing was found. Also, system restoring does not seem to help. I know a 5 year old cousin of mine was inadvertently playing with the computer aimlessly and may have done something, but I am not sure. I've cleaned the registry and defragged the hard drive to no avail.
Can anyone help me out? I'd really appreciate it.