Strange typing phenomenon on Vista Home Basic

One of my Vista Home Basic systems exhibits some quirky problem. Every time I type a tilde ~, it doesn’t show up until I type another tilde then both come up at the same time. This happens to double quotes ” and single quote ‘ as well. Typing other characters are all right. This phenomenon happens to all applications be they Microsoft Office or Open Office, notepad or Wordpad, Windows explorer or Internet explorer. Is there some setting on the Vista I have mucked up? I have already swapped the keyboard and it made no difference. The XP Pro system on the same computer shows no such problem. Strange!