Please take note (I've been told) that XP mode may require certain hardware to run (processor, chipset, bios, etc... ). XP mode will not run on just any machine.
		
		
	 
For a while, this was true (mostly PCU dependent).  After a while, XP Mode was changed to support most any modern processor.  
Please note that XP Mode is not "dual boot", but a Virtual PC.  There is a big difference.  It is a virtual 
Windows XP 
computer running inside your 
Windows 7 
computer.  It is pretty cool.  
Basically, you just get a window that looks like a whole new 
computer inside it, with 
Windows XP running.  You are still running 
Windows 7 and all your other applications, there is just a window running XP in a separate window (that can be shut down, minimized, maximized just like any other window).