fdisk/mbr

trying to fdisk a 15gigHDD with the fdisk/mbr command in order to eliminate any boot sector viruses, however as soon as i press enter it goes back to the A curser.
if i use just fdisk alone it goes into fdisk,but , after formatting with /u ,it onlygets to start loading win 98 then gives the following error messages :-SUWIN caused a stack fault in module SETUPX.DLL at 0019:3433.
any suggestions?