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.