Last time I had this problem, my Windows had decided it was perfectly ok for my Graphics and Sound card to share an IRQ. It's a irritating job, if you don't already know it, but worth a check

Click on Start ---> Settings ----> Control Panel ----> System ---> Device Manager.
Now right click on your sound card and select Properties
Now, from the far right tab (Resources) check your IRQ number.
Compare that with the IRQ of your graphics card, if they are the same, then one of them needs changing. if they are not, then it's not that

Ah well, took me too long to type it
