Check mousepad or whatever you use the mouse on.
Try cleaning the optical laser thing.
Uninstall ALL mouse drivers, old and new.
Try using it without drivers.
Reinstall MX518 mouse drivers.
Try moving the MX518 to a different USB slot.
Check for driver updates or patches.
Try messing around with the sensitivty on Windows AND the Logitech panel thing.
If all else fails, return/exchange, you might've recieved a faulty mouse.
But, if you lost receipt, try calling up Logitech for a replacement or warranty.
If all the above fails, try opening the mouse to look for any dust or broken pieces and see what you can fix.
I think that's all the possible ways to fix it.