If you're trying to install WinXP you could go into your system BIOS and change the load sequence by having the CD/DVD drive try to load first, that way you wouldn't necessarily even need a floppy bootdisk.
I'd recommend going to Bootdisk.com and grab the Win98 bootdisk as it comes with a driver for CD-ROM devices, of course you must boot with CD-ROM support. Then once at a command prompt you could switch to your CD/DVD drive - but of course your Windows installation is probably going to need to have a setup .exe file on it since it isn't automatically booting. Other than that, that's all I know about it since I've never had that problem happen to me and I hope it never does.
Edit:
Once thing I did forget is you can also run on your hard drive you're trying to install Windows onto is: fdisk /mbr
But I have no idea if that will do anything beneficial.
Your best bet is to probably browse/search Microsoft support articles for a proper answer.
Edit 2:
I forgot that a special version of FDISK is needed for todays larger hard drives!
1. Clean the CD with alcohol and a cotton ball then see if it works. Also, check for scratches.
2. Boot to a: - Attempt to boot from a boot disk that has CD-ROM support. Test the CD-ROM drive. If the drivers are properly loaded on the diskette, however, you still receive a problem, it's a good possibility it is a hardware issue; attempt to reset the cables.
3. Check cables - If you have moved or installed anything recently into your computer, or booting from A does not detect the CD-ROM, it's a good possibility the cables may not be connected or are loose within your computer. It is recommended that you open your computer and reseat the cables connected on the back of your CD-ROM drive and connected to your Motherboard.
4. Check drivers - Verify you have the latest CD-ROM Drives