Someone accessed my computer, used it for e-mails, changed files around, general mess. Lost AOL privileges due to heavy e-mail useage. Been told that someone can get in to my full time DSL line use my IP address, password, screen name, etc. to go around pretending their me.

Is this true?

Where can I get some solid reading material or web sites about how someone can do this. I suspect a former employee/relative who was in charge of my office computers did this somehow, but how can I prove this?
I need solid info to bring to my lawyer who knows nothing about computers and believes that no one can hack in and use someone's machine, e-mail accounts, and files.

Any help or direction would be greatly appreciated.

Thanks in advance.