Why does my PC think I have two copies of Windows?

The latest entry at Ask Dave Taylor is an interesting one, offering quite detailed instructions on how to solve the curious problem of someone's PC thinking that there are two copies of Windows installed on the disk, even though the person submitting the question was sure that there was really only one copy on their computer. The answer is from my friend and colleague Jim Boyce, a highly-lauded computer book author, and it demonstrates his usual accuracy and clarity:

    PC thinks I have two copies of Windows XP?

If nothing else, being able to salvage the space wasted by a bad Windows install is worth the effort.