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?

>>6

That's not what I mean...what I mean is that I also have to register the the OS w/ Microsoft...but the problem is that it says that my product key is invalid...:\  This is the first time I've ever had a problem like this.