Some additional points to investigate:
1) a hardware keystroke logger, which is a device that is physically connected to your computer and logs everything you type. Obviously a person would need to have physical access to your computer for this.
2) someone knows enough about you that they can pretend "oh yeah, I lost my password" and then successfully answer the necessary questions to get it reset, like your mother's maiden name, favorite pet's name, or whatever's required for that.
3) perhaps there's a keystroke-logging malware on your computer that your antivirus software does not recognize. Try making an
Avira Rescue CD, boot the system from it, and run a scan to get a "second opinion."
4) maybe you use super-easy passwords? Don't use a word found in any dictionary. Don't use anything that's obvious pattern-wise either, such as
qwerty or
12345678 or whatever.
Xodus01@AT would be nice and strong... it's sufficiently long, and it's non-dictionary, contains upper-case and lower-case, plus a symbol and numerals.