It sounds to me like the PO had configured a keyboard shortcut, that you are occasionally (and inadvertently) invoking.
Control Panel > Accessibility Options is worth a rummage.
Otherwise look for an item in the Startup folder, or an application set to run at startup via registry - might even run as a service!
Also look to see if a 3rd party application for the magnifier is running - check the installed programs, and inside C:\program files.
SD