Personalized systems are a response to the increasing number of resources on the Internet, but can be difficult to create. In order to facilitate the design and creation of such personalized systems, we aim at formalizing them. The situation calculus is a logical framework that has often been proposed to model web applications and even personalized ones. However, the details of its use are much more rarely explained. In this paper we will show that it is needed to carefully consider which variant of the situation calculus to choose. We will precisely show why we want to use the so-called guarded action theories. We explain why and how it fits into an architecture. We introduce two scenarios of personalized applications to illustrate this choice.