Argonians are not cold blooded. No lore supports this and circumstantial evidence such as the level of activity seen from Argonians both in-game and in the recent novels indicate they have some sort of mechanism to regulate body temperature. I.E. They do not spend the better part of the day basking in the sun, as is required from cold blooded creatures of such size. They may be facultative endotherms or selective endotherms, but not exotherms.
A developer quote on Argonians, Hist, and their relation can be found here: http://www.imperial-library.info/content/forum-archive-mark-nelson