Try updating your video card drivers. Check your IRQ for conflicts with the soundcard and video card. Windows will not report a driver conflict, but neither the soundcard or video card should be sharing an IRQ.
To see the IRQ, open Device Manager (right click on My Computer > Properties > Hardware tab > Device Manager button), View > Resources by type, the expand the IRQ tree by clicking on the "+". Video and soundcard should NOT have the same IRQ.
If sound shares with video, uninstall the soundcard in device manager, turn off the computer, then move the soundcard to another pci slot. Turn on the pc and wait for drivers to install - you might have to re-install soundcard drivers. Check again for IRQ conflicts.
With regards to onboard audio, there is ABSOLUTELY no way you can change IRQ assignments. That's why serious amateurs and pros use discrete soundcards, either internal (PCI) or external; and we don't use consumer oriented soundcards either, such as soundblasters, which are not intended for serious recording.