Chris I don't really find your term "secondary OS" useful. I'm not sure
what it entails, but I think you mean your main OS has been XP with all your
settings and programs and Vista would be the one you want to now try out.
The answer to your question is as you have been told 1) Install Old OS first
and I mean the older OS XP first. 2) You can burn the iso if that's what
you're doing and run setup from XP. You'll get as Dennis explained a menu
going to Vista by default and the choice to go to Previous OS. You
absolutely will keep your XP and all old settings on what you call your main
OS because you've been using it a while and have settings and apps on it.
3) You need to make a partition for Vista and in Vista setup you get the
option to "customize" and select your drive or partition for example if XP
were on C:\ you can select the other driver you have made E:\ or whatever.
4) If you run the setup from XP, and if you do the iso burn in XP it will
show up as setup on the XP desktop or if you put the DVD in at a later date
while on XP. This will insure you keep the same drive letters. 5) After
Build 5472--a build before 2 other builds that some people used, and
currently RC1 5600, some people noticed that they could not run setup from
XP and could run it fine with a restart. If you don't run from XP, the bios
will dictate the drive letters and change them. No big deal there. It will
not impact XP. When you boot to XP, the drive letters will seen as your
original drive letters. If you search dual boot, many of us have directions
in here we've posted for months.
Vista X86 is about 2.7 GB for RC1 and it puts about 7GB in ancillary files
on your drive. It also will install important files to C:\ or your XP drive
so on that you will need about 350 MB of free space. Then depending on how
many programs you install, you should have from 20GB to 40GB for Vista and
since you have so much space give it about 50-60GB.
You need to make a partition for it from the extra real estate you have, and
if you don't have a 3rd party partition maker/manager like Partition Magic,
then you can use
www.ranish.com and it will work well.
Here is one set of directions:
How to Dual Boot Vista
http://www.lifehacker.com/software/...-boot-windows-xp-and-windows-vista-179906.php
Threads for perspective
http://digg.com/software/HowTo_dual-boot_Windows_XP_and_Windows_Vista_-_Windows_Vista_Beta:_
https://blogs.msdn.com/dmahugh/archive/2006/06/15/632821.aspx
Get the Download of Vista RC1/Read this:
http://www.microsoft.com/windowsvista/getready/preview.mspx
This takes you to the download page:
http://download.windowsvista.com/preview/rc1/en/download.htm
As Dennis said, if you have any specific other questions, there is a lot of
great help here.
Good luck,
CH