if you purchased Windows 7 Professional or Ultimate, you are entitled to run 1 copy of Windows XP mode virtually on your machine. That's it. When you want to use Windows XP, Vista or Windows 7 as virtuals, you need to obtain licensing or use trials.
As others have said, you get 30 days and you can do the reset 3 more times to get a total of 120 days without activating on Windows 7. So, that should cover you for some time. You would then simply reinstall at the end of the 120 days and start again.