Operating system on my laptop is winXP. I had a HD put in with more memory, and the HD worked fine for a while, but now when I start my laptop, I just get a message on the screen that says 'Insert OS disk and press R to repair' I did that several times but I just keep going back and getting that same message again

How do I fix it

Sorry to say but it sounds like the hard drive has died on you, so replacement is your next move.

If your laptop came with recovery software or method, give that a try