All you need is to rewrite Master boot record so that it won't show Vista in boot option and delete or format partition where Vista is installed.

For that .
1. Boot using Windows XP Cd,
2. Press R when it ask for to choose you the option you want, then you will be taken to "REPAIR" mode.
3. Now you will have to type.
Fixmbr
fixboot

And you are done.

Second Method will need your Vista CD
1. Start your computer selecting Windows XP from the boot menu
2. Insert the Windows Vista installation DVD into your drive and close the menu that might open
3. Go to START > RUN > and type: "e:\boot\bootsect.exe /nt52 ALL /force” (without quotes, and replace e: with the drive letter of your Vista DVD).
4. Restart your computer and boot into Windows XP
5. Format the drive where Windows Vista was installed on
6. Remove two files (Boot.BAK & Bootsect.BAK) on your XP root folder (e.g. C:\)