Hard Disk Drive. There is usually a couple of lights on the front of the computer, one is steady for the power light and the HDD light comes on when you access the hard drive. If you go into MY Computer and double click on your C drive, watch the front of your computer and see if the a light flickers-that would be your HDD light.
If you do have a HDD light, watch it when you hear that noise, and if it seems to be making noise when the light is flickering, it's your hard drive.
Right click on your My Computer icon and choose Properties. Tell me how much RAM you have. It should be towards the bottom of the window that comes up. Is this a new computer or did you just install XP on an old computer?