I have had issues like this with yahoo email accounts and have asked Yahoo about them. Yahoo have told me that it is not an issue with either my PC or my friends but with Yahoo themselves. It seems what happens is that your contacts list are hijacked and sent emails that seem to come from you. Apparently the only way to solve the issue is to either change your email address or delete your contact list.
Mr Alba andI haven't been able to access either of our hotmail accounts for at the past couple of days. Me, I couldn't care less, Mr alba needs that one for his work :(