It's quite simple to get the second instance of Windows out of the boot menu:
Caution - you could make your system non-bootable (until you reinstall) if you mess this up.
A normal XP one would look similar to this:
multi(0)disk(0)rdisk(0)partition(1)\\WINDOWS="Microsoft Windows XP Professional" /fastdetect /NoExecute=OptIn
One with multiple listings will probably have other lines like:
Or whatever. Google "boot.ini" gives us:http://support.microsoft.com/default...;EN-US;q289022
He may have chosen to do this parallel installation so that you could browse the old folders and retrieve any data that you may need or want from the original installation...he may have just forgotten to format, or not had the balls to
Personally, if you have all your data, I would say boot off the XP CD, and reformat/reinstall.