Question about Outlook Express

Hello there!

I've stopped using windows live messenger for a while now, but someone called me and said that I've been sending her massive amounts of spam via Windows Live messenger.

So, I went over the usual steps:

I chacked wether I can log in  - I could
    There was someone logged in from "HOME-PC", whic is not the name of my PC, so I logged them out, but after a minute or so, theve come back and logged ME out. I tried again for a few times with similar seults.

I examined my live account for
aliases
forward emai addresses
conact email addresses
unauthorized skydrive devices
linked accounts - I wonder if I can see wther I have a parent account(s) attached, if not, that may be a way someone is controlling mine

I found these to be in order, as in, none was changed from what I remember and I'm completely certain that I have full control over the email address I used there.

I changed my password, but even seconds after that, when I singed in and logged out the other user, it came bacck immediately. I double checked the things abouve, but still found nothing unexpected, changed the password again, but with similar results.
It's as if the "hacker" managed to dodge entering my password.

I also tried to close my account after this, but received the "temporary problems" error when clicking on the close account link (from the appropriate menu on my account page). Then, searching these forums, I found another link, (posted by several moderators) tried that, and it seems to have worked, but when I checked from my sister's windows live messenger (and her account), I still showed up to be online and sending spam.

Any help would be greatly appreciated, thank you!

Best Regards,
B.F.

EDIT:
Of course, I use antivirus and firewall software, keep them up to date and scan for viruses frequently. In the past few years, my PC has been virus free.