Can't Switch Mouse Buttons

Here's a problem that someone showed me today on Windows XP. The problem: can't switch primary/secondary mouse button functions (ie. can't switch mouse from right-handed to left-handed). The background: One Dell laptop with Synaptic TouchPad and PS/2 mouse plugged in with USB adapter dongle. The person has switched the layout of their room, putting their laptop on the other side of them, so now, after several years of using the laptop, they want to change the "handedness" of the mouse. Their level of ambidextrous abilities is beyond the scope of this post, and why any right-handed person would suddenly want to switch is not my point. Going into the mouse settings, as per Microsoft's instructions shows that the "Switch primary and secondary buttons" checkbox is disabled. Of course this Google search comes up with nothing. At first I thought this could be due to conflicting software, and perhaps he had Logitech or similar software overriding Windows' functions. He said he didn't have any such software, and the generic brand name printed on his mouse, combined with the lack of any such program in the Start menu confirmed this. My next thought was that Microsoft or some other software couldn't handle having the buttons switched with the Synaptic pad turned on, ie. switching the buttons then touching the pad would be counted as secondary click. Disabling the touch pad did nothing for the ability to switch mouse buttons. Then I remembered (with some refresher from Google) about the command "rundll32.exe user32.dll,SwapMouseButton" but according to this page, that command is not a toggle, and not reversible through the same command. Seeing as how I didn't want the "client" to be left stranded in lefty mode, I didn't follow through on the command. Then I thought maybe XP was having problems with the PS/2 to USB adapter (despite the fact that I have changed handedness on Windows 3.1 with the old serial mice). Grabbing my USB mouse (Microsoft brand, no less), we unplugged his mouse, plugged in mine, tried to do the switch, then restarted and tried again. Then we logged on as system administrator in Safe Mode (thinking, maybe he didn't have permissions somewhere), and still nothing. I have no idea why he can't switch mouse buttons, after having tried all that. I narrowed it down to one final problem: "You're using Windows."