The paper presents a new tool to be used by architects and engineers for an optimized selection of windows in dwellings. The tool has been developed using Microsoft Office Excel 2007 and VBA and aims to be user-friendly and flexible, so it can be suitable for different levels of user expertise. The tool organizes the process of selecting windows in four different stages. A first stage, in which the user is able to compare the energy performance of different individual windows (varying in configuration, size and component performances) using the concept of net energy gain. In a second stage, the user may define a combination of windows to be used in a dwelling and calculate their energy use on a seasonal basis, taking into account the utilization factors of the solar gains and heat losses for the heating and cooling seasons respectively, according to (CEN, 2007, prEN ISO 13790). The main purpose of the third stage is to evaluate the indoor environment of critical rooms of the dwelling in order to verify whether or not the windows solutions selected as having a good energy performance also allow fulfilling the indoor comfort requirements defined in( CEN, 2007, EN 15251). The method used is the simple hourly method defined in (CEN, 2007, prEN ISO 13790). In this stage the indoor temperature and the heating/cooling energy demand are calculated for every hour of the year for each critical room of the dwelling. The fourth step consists of a simple economic evaluation for the combinations of windows previously selected, based on cost of conserved energy during the dwelling lifetime. Based on the overview of the stages previously mentioned, the optimum windows solution can be selected for each specific dwelling. In this way, the full potential of optimizing the energy performance of dwellings by optimizing the selection of windows can be used in an easy way.