I was able to install SP1 on my XPS 410, but I had to go through a few steps first.  People have posted that they have gotten it to work, but I don't think they know why except they reloaded Windows and didn't update their drivers to the sigmatel.  Since I have not seen a definitive step-by-step here it is.  Hope this helps some other people, like me.

Background:

The sigmatel drivers installed on XPS 410s and XPS 420s for the onboard audio conflict with SP1

http://support.microsoft.com/?kbid=948343#method5

IF you are not able to update to SP1 because SP1 is not in your Windows Update and you have the sigmatel drivers from Dell here is an option.

Overview:

You are removing the sigmatel drivers and audio program and installing generic drivers.  I have not found any issues with doing this, but IF you have an issue afterwards please post.  Please keep this thread on track and with details to help others.

Steps:

1) Access Device Manager

2) Expand Sound, Video and Game Controllers

3) Right click on the Sigmatel HD Audio drivers and uninstall - when you have the option make sure you check DELETE drivers

4) Access Control Panel - Uninstall programs

5) Locate Sigmatel HD Audio (or anything Sigmatel) and uninstall

6) Restart your PC

7) Windows Vista should load generic drivers for your onboard audio.  I ended up with High Definition Audio Device (drivers dated back to 2006)

8) I had to reboot again (wasn't able to get the SP1 pack by trying to check updates still)

9) Once rebooted SP1 should be available in Windows Update - I had to click "Check for Updates" twice to get SP1 to show

No issues since and the sound appears the same to me.

If you are in THIS situation (not that you reloaded Windows) and these steps work, please post that it worked, and your system specs - if not, please post your specs - to help others.