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.
More about :diagnostic boot selected dell latitude d620
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.