In XP go to START>RUN>MSCONFIG then go to the BOOT.INI tab. Click check all boot paths and it should error out asking you if you want to remove that selections, select YES. If it does not ask you this then you can manually edit the boot.ini file itself located on the C:\ to take out that extra line.
or if you dont want the chance of really really screwing up ur computer you can right click my computer, click advanced, click setting under startup and recovery... u can figure it out from there.
Start > My Computer >Right Click > Properties > Advanced > Startup and Recovery > Edit
This will open your boot.ini file. From there
copy and bast the hd or partition you want to be defualt in the default=".............."
[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 /NoExecute=OptIn
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Backup" /fastdetect
there is a drop down, he wont ever have to actually edit the actual boot.ini. imo, someone that doesnt know what they are doing would benifit greatly from a gui rather than an actual edit.