Reuse old XP PC HD as USB drive. Choose to boot from it on new PC?

My old XP PC power supply died. I want to take the HD out (from which it booted into XP) and put it in a USB external case. If I then plug this into my Windows7 PC, can I choose to boot from the USB drive, so that I can have an XP computer when I need it? This would be a real help to me since I have some old software that will not run on Windows7, and I really need to use this old software.

NO, not as a USB external.

http://www.sevenforums.com/tutorials/8057-dual-boot-installation-windows-7-xp.html?ltr=D
http://www.sevenforums.com/tutorials/8247-windows-xp-mode-install-setup.html?ltr=W

No it would not not work that way..
You can install windows XP on the same hard disk on which windows 7 is but in a different partition.
If you do it this way then when you would turn on the system you would get an option to choose in which os to boot.
Moreover if you have some software's that would not work in windows 7 you can try the "Windows compatibility mode" and it might make the software's work in windows 7.

What about using Windows Virtual PC. I haven't use it yet under Windows 7, but I tried it on XP.

I have tried compatibility mode but it will not run at all. Another thing I tried was to install an XP virtual machine, however I don't have the original XP install CD(s), only the upgrade from Windows ME, and it tells me it's not a valid registration code, so XP won't continue to install on the virtual machine.

If I install the old HD as a second internal drive, would the new PC have the option to boot from that at startup?

If you have Windows 7 Professional or higher, both XP Mode and a complete virtual machine containing XP with SP3 are a free download from Microsoft.
Otherwise, if you do have a valid XP key, you could try to find .iso images of XP online and use them to install it.

Using the old HD as a second internal drive may work provided everytime you want to use the old one, you have to change the old one as your primary boot and the Win7 disk as a secondary drive in the BIOS. This is of course if your BIOS is capable to do that, or else you may need to open everytime the case and physically change the dip switch settings on the 2 drives.

Corazon, I have Windows 7 Home Premium 64 bit only. The old HD is in my old PC right now. It originally shipped with Windows ME, but it was ready to upgrade to XP as soon as it came out. I got the XP CD's at that time and installed XP as an upgrade from ME, but it seems that it didn't ship with a valid key. When I got XP installed on a virtual machine, I couldn't get it to start until I provided a key and that's when it said it wasn't a valid key for XP. I don't know what all that is about, and I don't have any original ME cd's because that was pre-installed when I bought the old PC.

Shouldn't you be able to do a clean install with the XP upgrade disc as long as you feed it the ME key? (I'm actually asking, I don't remember if Microsoft allowed it back then.)

You will need to insert your Windows Millennium CD.

Corazon and Theog,
I don't have a CD or a key for ME because it came pre-installed on my old PC. The XP upgrade CD package has a key, but the install procedure won't accept it. It says it's not a valid key and so I can't get past that point in the installation.

If you want to try this old XP HDD on different computers, then it makes sense to put in a USB case. If you wish to use it in only one system (a desktop!) then it is better to install it inside the desktop.

One problem with the old XP HDD on new computer is that the motherboard and the devices connected to it are different. The XP may fail to boot. You may try to boot it in Safe mode and let it install the generic microsoft drivers first and then install manually other non generic device drivers. You may download the XP drivers from the manufacturer's website.

If the XP HDD works on your new computer, you can get some help from this forum tutorials or internet to add an entry in the Windows 7's boot manager so that the system becomes a dual boot system.

Did you try to mount the XP HDD in the Virtual PC as the First HDD (without a virtual HDD) and try to boot? It may work! Then you don't have to install XP.

Rraod,

I haven't removed the old HDD from the old PC yet. And I suppose I will still need to get a USB enclosure for it, and then plug it into the new Windows 7 PC, since that is where the virtual machine is located.

TheoG, as I mentioned before I don't have any ME CD's because that was pre-installed on the old PC. The only CD's I have are the ones that I used to install the XP as an upgrade from ME on that same old PC. That's why I don't have a product key for ME, but only for the XP upgrade, and the XP install on the virtual machine won't accept that product key. But I don't even know if I will be able to print from the software I need, because it will ONLY print to a parallel printer, and my new system doesn't even have a parallel port to connect to. Looks like I may have to completely rewrite the software in a Windows7 compatible format.

TheoG, I don't have any ME CD(s). It came pre-installed on the old CD. I did buy the XP upgrade, so I have those CDs, but the product key is invalid on the XP install on the VM.