On right-clicking an executable ((Though not every executable. If you want to run something in the control panel, probably the most common place to want to runas, you need to shift+right-click. Several MSI files don’t do it, and nor do any apparent CAB files. Let alone the inability to directly open a non-executable file as someone else.)) in XP and choosing `Run As…’, you are presented with the below box.
How on earth is ‘Current user’ a reasonable default value? If I wanted to run it as me, I’d just double-click it…