This is caused by a change in the BIOS setup of your computer. Perhaps your BIOS has experienced some autochange during installation. Possibly could be you were making installation with different versions of Window XP CD that overwrite the files.
(a) Solving the problem at BIOS
Image of PS2 Keyboard
Step 1: Simply get another keyboard(if you were using PS/2 keyboard before you can add a USB keyboard).
Image of USB Keybaord
Step 2: After adding the USB keyboard try to put on your computer and start your installation. The installation will disable the first one and enable the other.
Step 3: So try to continue your installation with the enabled keyboard. When you are through with your installation you can reset your computer to any of the keyboard you prefer working with by choice.
(b) When there is other keyboard available
Step 1: Go to your BIOS Setup and simple enable your USB or PS/2 Keyboard.You can learn how to enter your BIOS here http://www.computerhope.com/issues/ch000192.htm and how to use your BIOS setup very well here http://www.pcguide.com/proc/setup/biossafe-c.html .
Step 2: After enabling both Keyboards and you are still experiencing same problem you might need to use Acronis Disk Management utility or a HDD toolkit to erase your HDD, partition and format it. And repeat this same step.
(c) Haven't partitioned the HDD for so many time
Step 1: It doesn't matter excepting when you are receiving an error message of, Caution: This hard disk may be infected by Virus. Visit this LINK to solve hard disk infected by Virus problem.