and keep your current XP installation.
Boot your computer in to Windows XP.
Ensure you have the Vista DVD image emulated or in the DVD drive.
Go to “Start” and “Run”. Type
in “e:\boot\bootsect.exe /nt52 ALL /force” (without quotes, and replacing e: with the drive letter of your Vista DVD).
Restart the computer, and you will notice the boot selection menu is gone.
Format the partition/drive where you had Vista installed.
Remove two files (Boot.BAK & Bootsect.BAK) on your XP drive’s root folder (C:), these were backup files of your previous bootloader, now no longer useful.
Optional: Restart to ensure it still works.
Use your partition software to merge your partitions together.
from click here
If you want to wipe all and revert back, then thats a different operation.