First thing would be to run a good anti-virus to make sure she doesnt have a keylogger or something else. Also keep in mind that email headers can be forged easily to falsify who the email is from, so it looks like people are getting her email even though it came from someone else. That doesnt mean your email is compromised.
Funny, I just received an email in my spam this morning from "Me" which is the the name of the same email account I was logged into and it was from some chick that really wanted to talk to me?? LOL!!
Seriously, if it's just that they are spoofing your wife's email address, it might even be happening from one of her contacts email accounts and have nothing to do with her computer. I have a sister and niece (that sister's daughter) that I needed to create their very own email address for them to use to write to me. It seems their computers where so full of virus's and other garbage that anyone in their contact list would have their addresses spoofed. It got really bad after a while.