This could be caused by the Windows sounds that normally happen upon bootup. Disable this (possibly in safe mode if regular Windows won't load) in the control panel (Start - Settings - Control Panel) by going to:

Win98:
Go to Sounds and choose No Sound and hit ok.
Then Multimedia, Devices, Audio Devices, and double-click on your pro audio card's listing. Checkmark the button, "Do not map through this device" and hit ok, reboot.

WinME:
Go to Sounds and Multimedia, choose No Sound.
Then go to Devices, Audio Devices, and double-click on your pro audio card's listing.
Checkmark the button, "Do not map through this device" and hit ok, reboot.

This will disable system sounds, and also disallow Windows (Media player, Internet Explorer, Sound recorder, etc.) from using your pro audio card. It will not affect any recording programs; they bypass this part of Windows.