A few more details on the spec of the HP computer would be helpful. Like model number the CPU, installed memory and hard drive size would be a good starting point.
Is there an operating system already installed if so what one XP?
have you used the Windows upgrade advisor or have you just brought a Win8 upgrade DVD and stuck it in the drive, if so do you get any error messages. Unable to help much further without more information, just saying an old HP desktop is not much to go on.