Either you have upgraded from Windows XP to Vista and need to install Windows XP again
Or you have installed both Windows XP and Vista but you do need to reinstall Windows XP without losing vista then you need to follow the instructions given below.
Boot your computer with Windows XP CD-ROM then follow the instruction to install Windows XP after you have installed Windows XP then surely you will not find the boot menu which asks you to select either Windows XP or vista. Do not worry Vista is installed but it is not showing in the boot menu.
1. Insert Windows Vista DVD and wait for loading
2. When the first screen appears click Next, in the next screen click Repair your Computer.
3. System Recovery Options dialogue will search for vista installation and tell you that some startup problem was detected so it will give you the option to repair select Repair and Restart
4. After restarting again boot with Vista DVD it will show you the option Windows EMS Setup, just hit Enter and proceed as above until you find System Recovery Options again now you need to click Startup Repair as shown below
5. The next screen will attempt to repair vista boot menu reboot after the process is finished. Next time you start your PC you will see the boot menu and there will be showing recovered after your vista edition. If the boot menu does not show then repeat step 4.
Now you will have only vista running, you need to restore xp in boot menu.
6. Download and install this program VistabootPro
7. Run Vistabootpro select ManageOS Entries tab, Click Add New OS Entry, In the OS Name write Windows XP in the OS Type write Windows Legacy.
8. In the OS Drive you need to see where have you installed Windows XP but you do need to see in Vista on which drive Windows XP is installed.
This is because if you have installed Windows XP on C drive then you need to select D drive where Vista will show you that it is installed on C drive.
9. Click to Apply Updates
10. In the last step you can select Windows XP as set as Default