To install a 64-bit version of Windows 7 on a computer running a 32-bit version of Windows XP
1.Insert the Windows 7 installation disc or USB flash drive, and then turn off your computer.
2.Restart your computer.
3.Press any key when prompted, and then follow the instructions that appear.
4.On the Install Windows page, enter your language and other preferences, and then click Next.
If the Install Windows page doesn't appear, and you're not asked to press any key, you might need to change some system settings. To learn how to do this, see Start your computer from a Windows 7 installation disc or USB flash drive.
5.On the Please read the license terms page, if you accept the license terms, click I accept the license terms, and then click Next.
6.On the Which type of installation do you want? page, click Custom.
7.Choose the partition containing Windows XP (this is often the computer's C: drive), and then click Next. (Do not select your external USB hard drive.)
8.In the Windows.old dialog box, click OK.
9.Follow the instructions to finish installing Windows 7, which include naming your computer and setting up an initial user account. You can use the same names that you used in Windows XP, or choose new ones.