|Which model is it ? That's probably on a label on the outside of it.|
Why do you think the hard drive is not being detected ?
Is it detected in the bios Setup ?
If you are getting a message "no operating system found" or similar when you boot the computer, that's a normal message when there is no operating system installed on the hard drive - that doesn't necessarily indicate the hard drive is not being detected.
Why do you you say the hard drive has no operating system installed on it ?
Did you or someone else you know delete the data contents of the drive,or are you just assuming it has no operating system on it because you're getting a message "no operating system found" or similar when you boot the computer ?
Do you have or can you borrow a full version OEM XP CD, Home or Pro, the same as whatever version is on the official Microsoft label that has the Product Key on it that is on the outside of the computer case ?