Open the Registry, go to Hkey_Current_user\Software\Borland\Delphi2 (or 3),
and set or insert the key "DefaultFont" with a value like "Tahoma, 9". This
is the new default font.
  HKEY_CURRENT_USER\Software\Borland\Delphi\x.x\FormDesign

and the item is

  DefaultFont                   "fontname, fontsize, fonstyle1, fontstyle2"

where x.x is your version of Delphi eg 3.0 and fontname is the desired
font eg Tahoma, fontsize is the size etc etc