Examining peoples’ affect and emotions over time and their effects on peoples’ behavior are ongoing endeavors in human-computer-interaction (HCI) research. This paper reports an experiment in which participants watched either positive or negative film clips on a tablet PC to enter a positive or negative affective state. Successively, they accomplished four basic system interaction tasks like changing fonts of an app on the same device. Results show that, in line with previous studies, peoples’ general valence ratings quickly reverted to neutral when starting the task accomplishment. At the level of distinct positive emotions, participants’ ratings of hope, joy, and serenity decreased after watching negative film clips. Moreover, amusement, love, and serenity decreased during the interaction with the tablet PC. Amongst the negative emotions, only ratings of sadness increased after watching negative film clips and decreased again after the interaction. Also, participants in the positive film group were slower in executing one of the basic tasks than participants in the negative film group. The findings suggest that only few emotions may be causal for peoples’ ratings of general affect. Results also indicate that negative emotions may help people executing standard tasks, in contrast to positive emotions. Implications for HCI design and research are discussed.