Pretty much like Fibbi said: You can't just remove Windows 2003.
The best thing is to boot another O/S from CD (i.e. Windows XP or Windows 2000 Professional) and make sure you remove, re-create and format the partition on your hard-disk, Just to make sure you get rid of all the old "junk" you will no longer need.
Shouldn't be a problem at all but good luck
