This paper reports results from a controlled experiment (N = 50) measuring effects of interruption on task completion time, error rate, annoyance, and anxiety. The experiment used a sample of primary and peripheral tasks representative of those often performed by users. Our experiment differs from prior interruption experiments because it measures effects of interrupting a user's tasks along both performance and affective dimensions and controls for task workload by manipulating only the time at which peripheral tasks were displayed - between vs. during the execution of primary tasks. Results show that when peripheral tasks interrupt the execution of primary tasks, users require from 3% to 27% more time to complete the tasks, commit twice the number of errors across tasks, experience from 31% to 106% more annoyance, and experience twice the increase in anxiety than when those same peripheral tasks are presented at the boundary between primary tasks. An important implication of our work is that attention-aware systems could mitigate effects of interruption by deferring presentation of peripheral information until coarse boundaries are reached during task execution. As our results show, deferring presentation for a short time, i.e. just a few seconds, can lead to a large mitigation of disruption.