Now I have no sound... does anybody know how to fix this...

Thanks in advance

You'll need to know your soundcard or sound chipset make and model for that computer. Often doing a Windows Update will find the correct driver that you can use; just make sure to a Custom update not Express.

Go to HP's site and get the sound driver for XP.
HP Support & Drivers

Frankly I think you made a mistake. The tx1000 was diesgned for Vista, and you may not be able to use all its features under XP.

But the bottomline when downgrading an OS is to make sure you have all the drivers you need. Some hardware designed for Vista will not work with XP.

Try here, HP isn't very helpful in this department (downgrading).
Realtek