Not sure if one could expect to find great Mexican food in NH, but you can.
I have been to La Carreta on Daniel Webster Highway (just north of Tully VW/BMW) a few times and every time it has proven to provide fast service, fresh tasting food all in a very clean environment.
I had the chile relenos (sp?) last night with an ice cold Pacifico and it was great.
On other occasions I have had the burrito grande (12"x3") which is packed with beans, rice, meat & lettuce) as well as the fajitas. I like how the have haddock fajitas there and not just the standard chicken, meat or shrimp. The haddock fajita is highly recommended.
My wife really enjoyed the margarita that she had and her only complaint was that the pico de galo was too hot for her.
For a family of 4 to eat there with two appetizers and a couple of drinks for the adults it ended up being $55 without tip. We also had quite a bit to take home which some has made its way into a mexican omlette this morning.
Overall I would suggest trying this place out if you find yourself in the Nashua, New Hampshire area...by the way...when we left at 7:45 last night...there were still about 30-40 people waiting to get in.