Problems with WinXP Pro

Alright, I'm alright with computers but not the greatest so bear with me just a little bit here.

I recently installed a hard drive on computer and installed Windows XP Pro on the machine.  So far, it's been running fine, it boots up quick, it works great...but my problem is that it doesn't recognize any of the hardware...so I don't have any sound.  I only found this out when I tried to play some music using Windows Media Player and I got an error prompt saying that no soundcard was detected.

What can I do?

No...it says that there is no default audio device installed...something along those lines...and it looks like everything under "Other Devices" in Device Manager isn't beind detected.  This pisses me off by leaps and bounds...and I don't really know what to do.  To top it all off, I tried registering WinXP online and it says that I have the wrong product key...but the product key is right under the damn machine...