HP Support & Drivers | United States
Here is the HP Pavilion a1000 driver page. You will have to decide which is yours.
It may be that HP has no drivers newer than Windows XP.
If you can't find a Windows 7 driver at HP, then the one supplied by Microsoft on the Win 7 disc may be preferable.
Or a Vista driver in compatibility mode, if there is a Vista driver.
Realtek is a possibility as a last resort.
Your true last resort is to get another sound card--either internal if it is a desktop PC or probably external USB if you have a laptop.