Are you sure that keyboard hasn't a hidden setting somewhere, or a key combo, that switches languages?
Also check the following...I am using Vista but the settings are almost the same in XP. (not sure what you have)
Double-check Control Panel > Region and Language Settings > Keyboards etc. > Change Keyboards and other input methods > Change keyboards > Advanced key settings > Change key sequence > set that at None so no key combo will alter language settings.
I think you are right - something somewhere is fooling SiteAdvisor.
By this logic, my brand new Microsoft Bluetooth keyboard is made in China, so does that mean I can expect an unpleasant surprise at some stage...I hope not!!
I'll ask to see if anyone can think of a reason for this if nothing above helps. Let me know.