Umm, first thing is first, don't have the launcher save your last used login username/password. Enter it manually, and go to the .minecraft folder and delete the file which contains the last saved login. It's possible that maybe a mod you use (in single player of course, or at least a mod that gives you no cheating advantage in game) is backdoored, or you maybe have a keylogger on your computer.
I suggest you take a hard look at the processes list of task manager and look for processes that have suspicious names. God forbid you have a virus that is encrypted by a run-time crypter because then the virus would actually decrypt itself and then inject itself into a perfectly normal windows process. If not then you would see the virus on your processes list in Task Manager.
Also it might be recommended that you request that the mods temporarily ban you from the server for security reasons until you get this taken care of in case a griefer decides to use your account on the server.
