Non-administrator user accounts and the user account control (UAC) approach of Windows Vista are two practical solutions to limit the damage of malware infection. UAC in Windows Vista supports usage of lower privilege accounts; a UAC prompt allows users to raise their privileges when required. We conducted a user study and contextual interviews to understand the motives and challenges participants face when using different user accounts and the UAC approach. Most participants were not aware of or motivated to employ low-privileged accounts. Moreover, most did not understand or carefully consider the prompts. Keywords Usable security, Least privilege, User account control ACM Classification Keywords H.5.0 Information Interfaces and Presentation:General; D.4.6 Software: Access controls, Invasive Software General Terms Human Factors, Security