Did you use the MIXER (little speaker on the taskbar) and disable what you could? A common screecher is when the microphone is enabled.
As to disabling the onboard sound, it's usually a BIOS setting. Such is not standardised so you get to poke around or read the motherboard manual.
When I turn my speakers on they started emmiting a squeeling/raster sound, like a high pitched motor. If I turn the computer off it stops. When the computer comes back on it starts again. Changed speakers and same problem. Checking thru Device Manager all ok. Mic input seems to be fine. This is an E-macaine running XP AMD Athlon 2+Gig, 60 gig hard drive, current on all updates. My guess is my on-board sound has gone bad and I need to add a new sound card. Any other ideas or does this seem reasonable? If so how do I disable the on-board sound so a new sound card will work with out interference?