Hmm here are some more ideas;
Hold F8 while booting computer, should get a boot menu. Select the second option from the bottom (this should then drop straight into DOS without loading dosstart which may include some odd extra drivers).
If that fails try the same procedure taking the very last option, which loads DOS clean (without dos start etc..)
Otherwise, and as the very last resort, format a 3.5 disk and include system files. If you still cant access dos then the problem may be caused by the firmware
If that fails remove all cables from the computer, open window .......and examine the aerodynamic properties of your computer case.