How to turn on Intel VT in MacBook Pro 2011 (MC721LL/A)?
It should be turned on automatically. If you go into the settings for the Windows XP virtual image, then click advanced, you'll see a field for Preferred virtualization engine. It should be set to "Automatic" by default, but if you want to force Intel VT, you can select one of those options.

If you're lagging, something else might be to blame. Does Console say anything and how much memory do you have installed?