From your question I'm guessing that PC's aren't your forte.
Do you know any computer techies? I'd suggest asking them for help.
I guess you've replaced the hard disk in your PC, reinstalled XP only to find you have no sound. There could a number of reasons.
Did you plug the speakers back into the right socket? It's usually the green one.
Open the Control Panel and find 'Sounds and Audio Devices'. Is mute ticked? Is the volume slider over to the right?
Right click on 'My Computer' select 'Manage' and then higlight 'Device Manager' in the left hand pane. Is 'Sound, video and game controllers' listed? Can you see the sound card listed?
Are there any Unknown Devices listed on the right? If so then it's quite likely that one might be the sound card. You'll need to locate the sound drivers which should be on one of the CDs you got with the computer or on the manufacturers web site.