If you have installed vista as a multiboot sytem, then you can do the below: Otherwise you have to do as tom19511 said:
Uninstall Windows Vista on a multiboot system
Applies to all editions of Windows Vista.
You cannot uninstall Windows Vista. However, you can change the code that starts the operating system (often called the master boot
code), and then remove Windows Vista from your multiboot system and return to Windows XP or Windows 2000. You will need your Windows Vista installation disc for this procedure.
Before you begin, back up your programs, files, and settings. Removing Windows Vista is an advanced procedure that could result in losing information stored on your computer. For information about backing up data, search Help and Support for "back up."
1) Insert the Windows Vista installation disc.
2) Click the Start button , click All Programs, click Accessories, right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
3) Type [DVD drive letter]:\boot\bootsect.exe /nt52 ALL /force and then press ENTER.
For example, if the DVD drive letter is F, type f:\boot\ bootsect.exe /nt52 ALL /force.
4) Eject the Windows Vista installation disc.
5) Restart your computer.
Your computer will start in the previous version of Windows that you have installed.
6) Delete Windows Vista files and folders, such as the Program Files, Users, and Windows folders, manually.
Note: You can also remove Windows Vista by reformatting the partition where it is installed, but do this only if that partition is not the boot partition. Reformatting the boot partition might make your computer unable to start.