This seems like a strange problem with Windows XP. Some users are noticing scratchy, popping sound with their SoundBlaster cards and Windows XP..I have come to the conclusion that this is happening the most often on PC's that contain RAID setups such as a Highpoint controller.
The main fix I have come across is to install Raid drivers OTHER than those that shipped with Windows XP. For instance on my Raid setup, I went back to a older Windows 2000 driver and this has almost completely stopped my sound problems.