HELP! I need to boot to DOS

Hello all,

I need to boot to DOS, but this computer wont let me. I am helping my uncle to reformat the hd and do a fresh clean install of Windows. Right now its Windows 98. I try to dos by using the Windows 98 Boot disk and "Restart in MS-DOS mode," nothing works. Is there another way to boot to DOS? Please I need help ASAP. Thank you all.