Never having used Vista, I don't know what the "Boot.ini" file there looks like, but in XP, for a single boot it should look like this :
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect
NOTE : There should not be a space between "Micro and soft - and there isn't one when I edit the file, but this site's software is putting one there when I post.