I would suggest that your problem is likely one of four issues:
- Your sound drivers are incorrect/buggin/old
- Your sound card uses the CPU for sound processing and can't get enough cycles (rare in XP)
- Hardware accelleration is interfering with your sound card (already mentioned)
- You are using anolog/digital options without the proper anolog/digital equipment.
There are a few other things that could be an issue, but those seem to be the most common I have to deal with.