Normally you would simply select the OS you are running. Unfortunately Asus lacks any Vista updates that would work on 7 as well for the 32bit or 64bit Windows. The second image here the web address since the script continues to cause IE to crash for the Vista updated version seen at Download Asus P5L-MX SoundMax AD1986A Audio Driver V5.10.01.4160 - CODECSAD1988A/AD1988BThe AD1988 family of audio CODECs and SoundMAX software. - 4PCSoft.com
At least you will be able to find your back in case you run into any problems when the page loads there. It took 3 attempts here due to some script running on that page to post the link for it.
That was likely due to the IE security on here.