ANY Windows installation NEEDS an antivirus program, no matter what platform it's running on. There are a lot of free and inexpensive packages around, and Panda has a pretty good rep for such things. But do go ahead and activate once you have everything running (and wait to make sure you don't have to reinstall and start over). It should handle that for you after it asks you if you want to "register" your copy, as long as you have an Internet connection, but I'd wait until all the potential glitches (loading drivers, updates, etc.) have been taken care of.