Windows Vista

1. You must boot from the Vista installation disk .When the first window appears click Install Now (see Fig.1)
2. On the second screen click Don't get the latest Update(see Fig.2)
3. On the third you have to type your product key but,instead press Shift+F10 and a Command Prompt appears.
4. Type diskpart and press enter,you are now entered the diskpart utility.
5. Type list volume and press enter ,this gives you all the partitions or volumes,you should know on which one Windows XP is installed.
6. Type Select Volume 0 (where 0 is the number of the volume you want to shrink)You get "Volume 0 is the selected volume"
7. Type shrink .The process starts and volume 0 (the selected one)will be reduced of 50 % if the existing partition has enough free space.
8. Close the Command Prompt and keep going with Vista installation following the instruction on screen.

1. Click  Start Button and Right-click Computer
2. Choose Manage
3. Click on Disk management (see fig 4)
4. On the next window(see Fig.5) right-click the drive you want to resize, in your case there is only on drive, and choose Shrink Volume from the dropdown menu.
5. Next enter the space of disk to be shrunk, if you have a big Hard drive I suggest at least 15 Gig, for Windows, programs and some data. Click Shrink
6. When the process is finished, you have a section of your drive with a green strip at the top and the amount of free space inside,(see fig.6)
7. Right-click on the partition with the green strip and choose New Simple Volume.(see Fig.7)
8. The Simple volume wizard starts, you can accept all the default and change the label when you get to do it, and call this drive WindowsXP.The format process may take few minutes to finish depending on the size of the partition. At the end you'll have a new formatted volume where you can install Windows XP.
9. Go to start/Computer and you can see the new volume labeled WindowsXP (D:)(see Fig 8)

1. Insert your XP installation CD into the drive and reboot your computer to install XP.Don't forget on which partition you have to install it.
2. When the installation is complete,insert the Vista CD in the drive and reboot your computer from the CD
3. On the first screen click on repair your computer and not on Install Now (See Fig 9)
4. You now have to choose which Vista installation to repair but you should have only one,so click Next
5. On the next screen choose Startup Repair,and this automatically fixes the problem with the missing bootloader.Click Finish and the system will boot into Vista this time.
6. At this point to have the choice to boot from Windows XP or Vista you need to download and install EasyBCD that will do the job.You can download it Here
7. Install the program and launch it.Click on Add remove Entries
8. On Add an Entry click on Windows tab and select Windows NT2k /XP/2k3 from the dropdown menu and select the drive where you installed XP ,type Windows XP in the Name box and click Add (see Fig.11)
9. If you have done everything right,when you reboot you should have the option to choose which OS to boot from (see Fig 12)
   

You can easily change the default booting following these steps

1. Boot from Vista and right-click Computer and Properties from the dropdown menu (see Fig.13)
   
2. On Startup and Recovery click on Settings (see Figure 13)
3. On the next window click on the dropdown menu and choose which operating system you want your computer to boot by default and click OK.