this doesn't have to do with your BIOS, it is listed in your boot.ini file.
it is located at C:\boot.ini however, in order to see it you will most likely have to goto Tools > Folder Options > View.
make sure you have "show hidden files and folders" enabled and "hide protected operation system files" turned off.
you should be able to edit it using Notepad.
now here is the somewhat tricky part. when you boot up and it asks you to choose which XP you want to boot into, the order in which it is listed corresponds to how it is listed in the boot.ini file so be sure to remove the correct entry.
good luck.