Strange happenings in Live Mail and others?

I am sorry to be so vague, but my laptop has started to do some strange things.

I have been informed twice now since Saturday, by friends, that the e-mails that I have sent have been totally unreadable. They have been described as being in code? Yesterday morning I sent a single e-mail and when I checked to see that it was going along with the attachments by clicking on the 'Sync' button at the top of the page the window showed 6 e-mails going out? Is it possible that someone is 'piggybacking' my PC to send spam or other rubbish?

My OS is Windows 7 Home Premium and I am using Windows Live Mail. Everything is up to date as far as I know because I have auto-update enabled. My antivirus prog is Avast and that too should be up to date.

Also, I m using Firefox because IE8 was deemed to have a security problem, and several times I have been typing replies on forums I belong to and when I hit the 'New Line' key the page has swopped to a different tag?

Has anyone any idea as to what this might be?

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