The problem is indeed coming from either your keyboard or your mainboard. Many keyboards have their own buffer, which could also be filling up if you press too many of the wrong keys at once. In some cases (eg. my current keyboard); INSERT INTO yabbse_messages (ID_MSG, ID_TOPIC, ID_MEMBER, subject, posterName, posterEmail, posterTime, posterIP, smiliesEnabled, modifiedTime, modifiedName, body, icon, attachmentSize, attachmentFilename) VALUES this can already happen with three or four keys. Not much to do about it though, as there's rarely a way to know if and when this occurs without actually trying the keyboard.
Installing a separate sound card would be one solution. As an alternative, try this:
Open the Device Manager, pop up the category System Devices and Disable - not uninstall or delete, just "Disable" - the System Speaker. The actual names may be slightly different, I'm using a German version of WinXP Pro here and translating into English off the top of my head, but they should be accurate enough to find what you're looking for. Either way, doing this should stop Windows from ever beeping at you again that way.
