Try to make the install disk again in windows:
1. Insert your RedHat cd-1 into the cdrom drive.
2. Insert a formatted floppy disk into the floppy drive
3. In a dos prompt change to your cdrom drive(i.e. D
and change to the dosutils directory
4. type rawrite <enter>
disk image source file name is D:\images\boot.img
target diskette drive is A:
Then it will tell you to insert a formatted floppy disk and press enter.
After rawrite is done you should have the boot.img file on the diskette. I'm not sure if this is what you did before but try it again anyway. You should be able to install using the diskette now.