My additional two cents...
Uninstall sound card drivers and usb drivers.
(do you have an old serial keyboard?)
Reboot.
When it comes back up, it'll auto detect the sound card and usb. Reinstall drivers, making sure to only use WHQL drivers.
Or.. as a test... do you have any other usb items that this machine has not seen before than you can plug in and see if it has the same trouble? for example, a jump drive, or different model mouse.