Email Hijacker? - please help

I am hoping that someone can help me with this annoying and somewhat embarrassing problem I’m having with my hotmail account.
Over the last two months, on almost the same date (don’t know if that’s significant?) I have received an email written in a very friendly and familiar style that strongly recommends I visit the website of two (so far) “excellent” companies that offer all sorts of incredible bargains.
It turns out that these companies are based in China, and from what I can tell, appear to be in the business of offloading tatty, out-of-date electronic products. So what? It’s just junk mail and everybody gets that, but the trouble is that as soon as it hits my inbox it immediately sends itself to everyone on my contacts list – friends, business associates and just about anyone else who knows me, and of course from the way it’s written it comes across as my personal recommendation! Unfortunately I haven’t kept the addresses of the senders of these emails – my first instinct was to bin them – but they begin with a number, and then continue “Hey, how are you doing recently?…” and so on, then finish “ Regards “ and then there’s another number.
I guess I could delete my contacts list and keep the details on a Word document away from hotmail, but I’m hoping that one of you guys may be able to suggest a way I can stop this – maybe at the ‘mail in’ stage, before it can send out this rubbish “on my behalf”.
I am running Vista Ultimate with Explorer 7 and use a Belkin router with firewall. I also have Avast Antivirus Professional. Sorry this is all a bit ‘wordy’ but I thought I’d better include as much as possible. Many thanks in the hope of getting some of your expert help before the next instalment arrives!
(I previously started this thread in the 'Malware' forum, but was advised by TimW that "These are issues that really are not within your computer system...but rather your online email server. You need to post in the software forum where others who may have had this issue can respond."

 

I find that very hard to believe. Are you sure you don't have some type of malware infection?

 

Many thanks for your replies DomLuc and LauraR - I will follow up on the scans as you suggest. I would point out though that whether or not I open the message makes no difference at all to what happens nextl - in fact the last time it arrived I was at work and got a message there "from myself" because my Outlook email account at work is in my hotmail contacts list (if that makes any sense!). I hadn't even opened the original message that arrived in my hotmail inbox before it sent itself back out immediately to my entire hotmail contacts list - and this is the worst part of it! Spam I can cope with - we all get our share of that - it's just this immediate redirect to all my contacts (before I've even opened the message) that I really need to get sorted out.
Also I notice that both times this has happened I have got 'refused delivery' notification messages from some of my contacts - is this perhaps because they have software that detects that this isn't genuine and not really from me, and if so is this something I could use to stop the original message from arriving in my hotmail inbox in the first place?