i have Vostro 1510 and until the begining of this week i could use dual monitor option. i don't know why, but now when i connect monitor to the vga out port, the computer recognize it, but i see no input on it. i tried switch resoulotion, but it's not working. i uninstalled the vga drivers (nvidia 8400m, latest), and i could use the external monitor or the laptop monitor, but not toghether. when i reinstalled the drivers, the problem returned. in ubuntu live cd, everything work perfectly, so it's not likely that it's an hardware problem. i don't remember installing anything that may cause this. i tried to use combofix just in case, but it didn't help, so i restored back. unfourunely have no system restore points from the period when it worked.