MSN account compromised for spam: How did this happen?

I've recently been informed that my MSN hotmail account has been sending out spam messages over IM. I use the following protection:

Avira antivir realtime protection
On-demand spyware scanning with SVS and MBAM
Comodo Firewall
HijackThis

I use Firefox, with NoScript. The worst stuff I've found on my system since good old blaster has been tracking cookies, which get the boot every time I close down Firefox.

I keep my passwords in KeePass

What could have led to my account being compromised? I can tell a phishing site from a mile away and don't consider it a possibility. The only thing I could imagine is that my e-mail address and password were recovered from a hacked website that lacked password encryption, or a vulnerability in my IM application (Pidgin).

Can anyone else suggest where I should be looking? I am systematically changing all my passwords and updating my software. If I can't rationalize this somehow I'll do a reformat despite the clean logs from Avira, MBAM etc.

I really cannot believe that I've left a loose end somewhere. The more I think about it the more I think some 3rd party service that had my account details was compromised.

 

Do you mean that your msn account is actually sending out spam under your email address or does it only appear to be your email address when in fact the spam is being sent out from elsewhere.

If it is the second case, I don't think you can do much about this but get a new email addy. I am constantly getting spam supposedly from my own address but it's just fake. Somehow, somewhere my address was harvested ( and you can never completely prevent that) and is now used as a "sender". Annoying, especially if you have to explain to your friends or contacts that the spam in question has not come from you.

 

Hi beethoven,

Sorry I was a little too concise in the thread title. My account was sending spam messages to my contacts over MSN instant messenger. Have you ever logged into an IM client to have one of your friends send you a fake message along the lines of "Hi I thought you would like to see this http://spamwebsite.example.com" - that's what I'm referring to.

The only place that I think may have had my credentials is meebo.com, but there haven't been any security alerts recently... I'm stumped, I let Avira do a full system scan after posting last night and it came back clean. My credentials must have been leaked from a source other than my computer... which I suppose illustrates the importance of not using the same password everywhere!