Depending upon what model of PC you have, holding down a specific key whilst powering up the PC generally gives an option for entering the BIOS. The DEL key is probably the most common but I have had some PC's that want the ESC key or F1 key instead.(I presume that you want to get into the BIOS to allow a boot from floppy to cut ahead of boot from HDD).
FDISK is the utility that you need to remove the Linux partition. Boot into DOS from a floppy disk and run FDISK.
Alternatively you can run FDISK from within Linux.