This study investigates the effectiveness of computer-delivered hints in relation to problem-solving abilities in two alternative indirect instruction schemes. In one instruction scheme, hints are available to students immediately after they are given a new problem to solve as well as after they have completed the problem. In the other scheme, hints are only available as worked out problems after students have finished their solution. The instruction schemes are supplied by means of a web-based program, Physhint, which supports the development of strategic knowledge [Pol, H. J., Harskamp, E. G., & Suhre, C. J. M. (2008). "The effect of the timing of instructional support in a computer-supported problem-solving program for students in secondary physics education." "Computers in Human Behavior", 24, 1156-1178]. This program supports novice problem solvers while undertaking physics problems concerned with forces by providing hints structured in accordance with Schoenfeld's episodes [Schoenfeld, A. H. (1992). "Learning to think mathematically: Problem solving, metacognition, and sense making in mathematics." In D. A. Grouws (Ed.), "Handbook of research on mathematics teaching" (pp. 224-270). New York: McMillan Publishing]. An experiment was carried out in four schools in order to study students' use of the hints in both of the computerized instruction schemes, as well as the effect of different uses of the available hints on students' ability to solve physics problems. The experiment consisted of three groups. Two groups of students were assigned to one of the two instruction schemes and a control group was selected for the purpose of comparison. The results of the experiment show that both computerized instruction schemes are effective. Students working with the most elaborate instruction scheme show an increased use of their pallet of heuristics and algorithms in the post-test. Furthermore, the instruction scheme in which hints are available to students during problem-solving proves to be most effective when students show an increase in the systematic use of hints during problem-solving. This paper therefore provides an insight into how a computer program implemented in school practice can improve students' strategic knowledge. (Contains 11 tables and 4 figures.)