First thing to find out how to get into the BIOS. That varies between motherboard, alas. You should be able to find it in the documentation that came with the motherboard or the computer.
Most BIOS-ses display it on the bottom line of the screen, however, but not all do. It's a line like "Press del to enter setup." It could also be f1, f10 or escape, or another key. You press that key very early in the boot process.
In the BIOS you look for the boot sequence (exact location depends on the maker and maybe version; it should be in the documentation). Change it to boot first from a:, and exit the BIOS while saving changes.
Then put in the boot diskette, boot and run every DOS-diagnostic for the hard disk (c
that you can: fdisk /stat, chkdsk c:, scandisk c: and dir c:.
If basic hardware is OK and functioning, but things on the harddisk are beyond repair, you can set the BIOS to boot from cd first, and boot from the recovery CD.
Tell us your experiences if necessary.
Kees