Windows 7 - How install Dual Boot after Win 7 has been installed

I am using Windows 7 home premium. I would like to have a dual boot system so that I can boot in XP or Windows 7. I already have Windows 7 installed. I like it and it is working fine, however, I want to do some beta testing of Office 2010. Optimum situation would be to install an older copy of XP I have to a parition on an external USB drive. When I boot my Dell, press F12 and have it allow me to pick the partiion on my USB drive to run (XP).

I have googled Dual Boot Windows 7 but almost all the tutorials have to do with XP being installed first and creating a partition for Windows 7, the opposite of what I want to do.

I know enough to know if I mess this up it can have unpleasant ramifications. Just trying to find some detailed instructions specifically for what I want to do.

Download Virtual Box and install XP as a virtualized OS. No mess for partitions.

If you have 2 internal disks, you could install another OS on the second disk (first disconnect the Windows 7 disk) and then switch between the 2 systems with the BIOS boot sequence. That is how I run Vista and Windows 7 on the same system.
Another possibility is to use a virtual partition using Virtual Box. Here is a tutorial I once wrote up for installing Windows 7 in a virtual partition - but it works with any other guest system. I installed Ubuntu that way.
Tutorial: How to install Windows7 in a Virtual Partition How-To Geek Forums

Frankly, I try to stay away from double booting. It is always a mess with the bootrecords. But the above approaches are trouble free. With the virtual partition you have the advantage that you can switch between the two systems with one click. And if you don't want it any more, you just delete 2 folders at the host.

Edit: Frostmourne was faster - LOL.

MS works hard to keep its OS's from being installed on external drives. But there is an extremely complicated tutorial on the internet to do just that for XP.

I would tell you its impossible with Windows 7 but someone will beat me over the head with a rotting turkey leg. Let's just say Google has found no one who's done it yet.

What is your status, do you want to install it on an second hard drive, or another partiton on a hard drive, or do you want it to install on an external drive

You know how to install to external drive?

Although i've never done it, i have had my external HDD plugged in when i've installed on my primary drive and when i choose an advanced install, my external drive is detected and displayd as onan installto. Why would it detect and offer as a drive to install to if it wouldn't do it?

It will refuse to install to external.

MS puts a lot of trouble into keeping OS off of externals or else people could carry around their OS to any computer.

Guy in here last week was insisting it could be done, but couldn't show where or how.

As stated, it was eventually done with XP but it is one of the most complicated tutorials you've ever read.

That's the best, most efficient, and least complex method. You don't have your system specs filled in, so XP Mode may or may not be an option.

You cannot install an OS on an external USB drive. You can install from a USB drive if your PC allows to boot from USB - but running a system from an external disk attached via USB is not possible.
I wonder, however, whether it would work with an eSata connection. Anybody who has a disk connected via eSata may want to try that and report back.