Have you tried to boot from a DOS floppy?
Your PC may have a feature referred to as "Silent Boot" this give you a splash or blank screen in place of the POST (Power On Self Test) screens.
You may need to set a jumper to enable going into BIOS settings to enable the POST screens.
As to booting into DOS mode, what you need to do, is enable the boot option keys at startup.
I recommend using a system utility such as TweakUI to perform the changes required to see the Boot Keys.
The Code to enable the boot keys is in the file MSDOS.SYS
THIS FILE MUST BE KEPT TO THE SPECIFIC SIZE IN ORDER TO FUNCTION PROPERLY!!!
The statments to change are:
BootKeys=1
BootMenu=1
BootMenuDelay=15 (the number of seconds for you to choose a menu option before the system boots into Windows
If you want to boot into DOS everytime you start the PC, set BootGUI=0
Chas