I have a DELL Latitude D620 that since a month ago always starts with diagnostic boot selected. If I press Escape several times the machine starts up as ususal, but wit a series of high blip sounds. I can use the computer normally after start up. When I start the computer again (start over) it starts up with F2 and F12 setup as normal.
On the general tab, make sure normal boot is enabled. On the boot.ini tab, make sure no boxes are checked. Also, you only have 1 operating system under the [operating systems] header.