Installing Linux on a windows hd

[Roger]

A friend who is not computer literate at all has a Dell 1520 laptop 
which when new, Dell required it have xp installed. I insisted at the 
time that the hd have 2 partitions as I intended to install either 
Fedora or Ubuntu but that was years ago and I had forgotten all about it 
till today.
Macafee insists on updating but will not do so until she updates 
explorer which she never uses and knows nothing about. I have no idea 
about updating explorer either, never used it. I've googled but my 
google produces different results to her google search so I can't tell 
her to click on this or that option.

I do not know if her xp is sp1, 2 or 3 and have no way of finding out as 
all contact is by phone, she doesn't  have a clue about finding out, me 
neither, and she lives some 120 km away.

I would like to run her through installing one of the Linux systems on 
the second partition but am worried that on installing it may overwrite 
the windows MBR making her windows files, folders, etc useless, 
particularly Thunderbird and Firefox. She would not be able to reinstall 
windows without me spending a day on the phone. I no longer have the 
stamina for epic phone calls.

This person has no confidence with computers. Over the years I have 
talked her through windows problems but now I am lost as to how to 
tackle this one.

I understand how to get linux to clean install on the second partition 
and I think I remember about how to use it's own boot record not the MBR 
but have no idea how she would access linux with windows on the first 
partition. I would prefer to make Linux the default boot option without 
overwriting her MBR. I think this can be done as a bios option as I do 
on my desktop pc.
I think the best linux option would be ubuntu 12.04LTS so she doesn't 
have to update the OS.
Can someone direct me on the best approach for this as I would have to 
set aside 3-4 hours on the phone to walk her through a basic install 
before the file system updates, which can also take hours.

Help is most gratefully appreciated.
Thanks in advance
Roger