Will give it a shot to help you with your problem. Would of been a bit easier if we knew what Brand of motherboard and the cpu type.
First off you will need to get into your Motherboard Bios'. This is done by pressing either the "Del key or F2", depending on motherboard type. Once you are in the Bios' section you will need to find Boot Start-up order. You will need to have "CD drive" set as first boot-up option then followed by HDD and finally Floppy drive. Once that is done you will want to save what you have just changed when exiting the bios'.
Also before you leave the bios', put in the XP operating sysem cd into the drive, then leave the bios and restart computer. This should help you Install the OS.
Please let us know if you need more help and that you got it working.