Problem with “phantom” emails apparently sent by me being received by random people.

My email system (Windows 8.1 and Windows Live Mail) has been compromised in some way which results in a batch of emails being received by an apparently random group of people who in some cases reply to me - mostly saying "unable to deliver". The problem is that I have not sent the emails that are being replied to and they are generally sent in batches (3 times since March this year). They have my email address in the "from box" at the top and generally contain a link to a website with a message like - I have just made £380 in the past week! I have been in touch with BT who provide telephone/broadband/email and they have so far been unable to solve the problem or give me any feedback. I do not know who is pretending to be me or indeed why they are doing what they are. I have changed my BT password to no avail and do not know how to tackle the problem. I have Microsoft Security Essentials and Malware Bytes installed. Can anyone suggest a solution?