This found in comment 205 of the article (an even better way IMHO)By Doug Hynes
The following method has been working for me to add an XP boot to an existing Vista. It has the advantages of not needing to repair the Vista boot AND of having the XP system drive installed as C:
1. Create the available space as described in the article
(This was using either a complex Vista install boot sequence, using Gparted, or I would recommend the simple and quick booting BootIT Next Generation. The result was an unallocated space on the HD to next install XP)
2. Using Disk Manager from Vista, create a new partition in that unallocated space — don’t use the XP install to do that.
3. Still in Disk Manager, set that new partition as Active. WARNING: That means that the machine will now be trying to boot from the empty partition. That’s OK because the next thing you’re going to do is install XP from a bootable CD. If you restart and then change your mind, you’ll have to have some bootable utility to change the active partition again.
4. Boot from the XP installation CD and start the install. When you get to the step where you select the XP partition, you’ll notice that your new target partition is C! That’s because the active partition is always assigned that letter at this point. So your new partition will show as C and the existing Vista will show up as some other letter. So XP WILL be installed as C. Vista will remain C, too. Finish the install.
5. Once XP is running, copy NTLDR, NTDETECT.COM, and BOOT.ini from the XP partition to the Vista partition. This is required because the Vista partition will soon be the boot partition again .
6. Still from XP, use Disk Manager to change the Active partition back to the original Vista partition. The Vista partition’s letter will show up as something other than C, doesn’t matter, it will be C when booting Vista. Since the XP install never touched the Vista partition, NO repair is needed — reboot and Vista will startup again.
7. Use (NeoSmart) EasyBCD as described to add the XP boot.***(see steps copied below)
I can vouch from experience that this works very well. In fact you can have any number of Windows OSes all running as C using this method. You can also adjust drive letters using the HKLM/System/MountedDevices registry key. I’ve used this method to have 5 or 10 OSes installed in different partitions all at one time, and to restore various images to any partition and then fix the drive letters.
*** Step 7. addendum
Now we need to enable dualbooting with XP, and EasyBCD is the best application to achieve this.
(by the way – NeoSmart Technologies builds and freely supports EasyBCD and has guidance on doing this and other multi-boot scenarios including using GRUB, Linux, Mac OS X, OS2, Solaris, ..etc)
Launch the app and go to Add/Remove Entries.
Under “Add an Entry” and under the Windows tab and select in the Version drop-down list “Windows NT/2k/XP/2k3”.
Change the Drive to E:\ and the name to “Windows XP”, then click “Add Entry” and “Save”.
Vista & XP – EasyBCD
Reboot the system and you’ll have two entries in the Vista bootloader, and can boot into either operating system.
Vista Bootloader – congratulations mission accoomplished