I don't think so. You get the full OS until the Activation period has expired, then nothing but a request to Activate.
I know if you don't activate XP within the 30 day window, you're pretty much locked out on day 31. I also know that with some trial versions of certain software programs you can't access some areas or parts of the program do not work, until you register, acitvate, purchase the full-blown program - whatever. So my question is, are there some parts of the XP OS that do not work, or do not work right until you Activate?