If it is webmail, it is possible that someone get access to your account or password . You may type it in unsafe place.
My first suggestion is to change your email password. Update Norton and run full scan.Also contact your email Admin and inform them about this issue. They would invastigate and help you with that.
Also you could try following website:
For XP:
http://onecare.live.com/site/en-us/default.htmFor Win7/Vista:
http://onecare.live.com/site/en-us/center/whatsnew.htmAnd run full scan. It will detect and remove Spyware .