Chrissa,
Echoing everybody's expressions of sympathy, and Saxy is quite correct. You might be surprised (or maybe not now!) by just how easy it is to hack into certain email accounts, especially the ones you'd least suspect, like HotMail and Yahoo. GMail is little harder, but not much. Changing the password makes not the slightest difference...
The only way to prevent this from happening is to deactivate the account. This will immediately make it of no use to spammers because it'll no longer be "live". Live email accounts are like gold-dust to these scum, because they use them to make their spam appear to have come from a valid account, which helps the messages bypass spam filters.