I read somewhere that most cars that a are sold to high altitude areas are tuned to work well with the different atmosphere. I'm working in Colorado for the summer and I'm from Massachusetts so is my car going to run poorly with the high altitude or will the computer be able to compensate properly?

Your car's ecu will be fine, but just dont expect it to have the power it has at sea level.

Yup, lower oxygen levels at higher elevations will cause a drop in performance. ECU will adapt as best it can, you should be fine.