|Is it Windows 98 you are trying to install then? We need to know.|
It is probably seeing the C drive (HD) first and therefore skipping the floppy.
Go to the BIOS screen (Del) and ensure that the floppy drive is set permanently ahead of the C drive. Sorry I can't give more detail because all BIOS screens are different. If you should goof it, back out without saving (usually Esc key).
Alternatively you can use the simpler F12 option which will allow you to put the floppy drive ahead of the C drive for the next boot only. You then put in the floppy and reboot. If you need further tries repeat the F12 procedure.
Always pop back and let us know the outcome - thanks