Have you bought 8GB of the exact same RAM? Then have you correctly installed the 8GB of RAM? If you had 2 4GB sticks maybe one 4GB stick is not correctly installed. Next please go to Windows Task Manager, to open Task Manager click Ctrl, Alt and Delete at the same time then click Start Task Manager in Windows 7. Once Task Manager is open please go to the Performance tab and see if it shows 8GB of RAM there. Also go to Windows Control Panel, System and see if 8GB of RAM is listed in there.
Please screenshot the Control Panel window and the Task Manager window and post the images as a reply to this post.
Thanks