Embora o método recomendado seja instalar o XP e o Windows 7, não é necessário reinstalá-lo no seu caso.
Siga este guia (editado abaixo) usando uma ferramenta gratuita chamada EasyBCD .
Download and install EasyBCD. Click I Agree to the license agreement, click Next to install in the default location, and the installation wizard will do the rest.
Click View Settings.
Change the Default OS to Windows 7. The operating system to associate the settings with should be Windows 7 too. Select the drive on which Windows 7 is installed under Drive. Type Windows 7 in the Name box and press Save Settings.
Click Add/Remove Entries.
Under Add an Entry, choose the Windows tab. Select the drive on which Windows 7 is installed. Type Windows 7 in the Name box and press Add Entry.
Under Add an Entry, choose the Windows tab. Select the drive on which Windows XP is installed. Type Windows XP in the Name box and press Add Entry.
Exit EasyBCD and restart your computer to be presented with a multi-boot option screen for Windows XP and Windows 7.