I know you're a fan of macros and similar kinds of automation. Is there a chance that you had originally automated some kind of a task involving opening and modifying multiple files in MSPaint, and somehow a vestige of that macro is getting run each time it loads?
In a word... no. When I drill down to C:\Windows\System32 and double-click the mspaint.exe file, that's about as direct as I can get, and it doesn't work that way either.
In the event that nobody here has the answer, at what other venues might I appeal for help?
tanstaafl.