From what I have read, there's only two possibilities left:
? A hacker has somehow hacked into and took over your computer, including your mouse. You did mention finding a Trojan Horse before; it could be very possible that the Trojan Horse may have opened up a hole in your computer's security defences, allowing hackers to have access to it (and programs that do such things do exist). In this case, update all your virus and spyware definitions (which could be outdated), disconnect from the internet, and do a thorough scan. Get a free firewall, such as ZoneAlarm.
? It could very well had been faulty hardware or a lose connection, like what Alana suggested. If so, check your mouse's cable's connection. If it's a wireless model, you may need to check the batteries. You mentioned that you cleaned out your mouse; the mouse may had been damaged during the cleaning process. In this case, get a new mouse known to be in working condition and use it with the computer. If everything works properly, then it's just a problem with the computer mouse.
I suggest that you provide more information about your computer system before you do anything else though, such as the operating system, internet connection type, and the type of mouse you are using (wired or wireless and ball, trackball, or optical).