"The amount of system memory has changed..." I need help!

It started a week ago with a post-check beep, black screen with white text, and the following errormessage: "The amount of system memory has changed. Press F1 to continue or F2 to enter setup" So i enter the BIOS setup, and my memory has been reduced to 256 ddr RAM. I press esc to reboot, then windows start (alot slower than normal) and windows also confirms that my memory has been reduced to 256. I shut down my PC, and when booting again, i get the same one beep and black screen, with similiar errormsg. But when i enter the BIOS setup, it says 512 RAM. So i press esc, and start windows fine. This happens 3-4 times during the week, shutting down the computer and rebooting it works like a charm every time, until today. I got the same message, but now rebooting dosen't work! It's stuck with 256 RAM, and nothing seems to help. I've tryed the following: 1) Starting the PC, without battery, CD-ROM and every unnessesary plug-in removed. Diden't work. 2) Resetting the BIOS with alt+f in the BIOS menu. Diden't work. 3) Running all kinda tests and virus scans, they all detect 256 ram and no problems other than that. I've googled some anwers, but alot of them are on desktops, and they have the advantage that they can remove their RAM. At this current state i do not know how to remove the RAM from my laptop, and i have no idea what to do. Hope you guys can help!