My 'gut instinct' is that the problem lies in when you are calling _MOUSEINPUT

I tried to fix your program but to be honest I couldn't get it to muck up on me like it does for you, so it was more of an educated stab in the dark. Don't blame 'your' computer or 'your' version of Windows 7. Timing is a very interesting thing and it varies between computers. A tiny difference in timing could easily (and I believe does) account for the differences you and I are seeing, but if it was coded correctly there wouldn't be any differences to see.

My 'gut instinct' could be wrong about this, but that's what the test programs I'll give you will try and find out.