That's the Virtual PC manager. You can have more than one Virtual Machine configured and that's the interface to manage them (you can change settings, how much ram is allocated, etc.) as well as starting and stopping them.
To start the Windows XP Mode that you set up, you can:
- Click on Start -> All Programs -> Windows Virtual PC -> Windows XP Mode
- Open the Virtual PC Manager and double click the machine you want to start.
- Open the Virtual PC Manager and right click -> Open to start the machine.
As you install programs in Windows XP Mode shortcuts will be created in your Windows 7 start menu to launch them.