From the sound of it, I’m going to assume that you are running a dual boot configuration also known as a multiboot configuration between Microsoft Windows XP and Windows 7. When it comes to designing a system with both Windows 7 and Windows XP there are two ways to accomplish this task. The first method is to create a dual boot, or multiboot configuration between Microsoft Windows XP and Windows 7. The first step is to ensure you have a separate location for each operating system you intend to use. In this scenario you will want to have one partition for Windows 7 and a second, separate, partition for Microsoft Windows XP Professional. Just as an added tool, Microsoft has a helpful video of how this process is performed here. Another great resource is the multiboot step-by-step guide Install more than one operating system (multiboot).
However, with that said, perhaps the easiest way to run Microsoft Windows XP within Windows7 is through Windows XP Mode and Virtual PC just as another member of this thread pointed out. Windows XP Mode and Windows Virtual PC, available on Windows 7 Professional and Windows 7 Ultimate, allow you to run multiple Windows environments, such as Windows XP Mode, from your Windows 7 desktop. For those who do are not currently running Windows 7 Professional, Windows 7 Enterprise, or Windows 7 Ultimate, there is an easy upgrade path to Windows 7 Professional or Windows 7 Ultimate through the Windows Anytime Upgrade program.
You will also want to ensure that the boot order for both hard drives is properly configured. You will likely want the Windows 7 hard disk drive to be the boot volume as you are repairing the boot record on it. The secondary hard drive would be the remaining Windows XP hard disk drive.
Performing these steps will allow you to have a selectable boot menu containing options for both the Windows 7 operating system as well as the Windows XP operating system.