They are either running the default email program or they are just running Outlook because they know that if you have Windows, you have Outlook.
If they are just running Outlook (or some other email program), there is nothing you can do.
If they are running your default email program, then you need to make sure that the email program you want to use is set up as your default email program. Usually installing that email program, will put it as the default. I would think AOl would do this but Yahoo probably will not.
Outlook Express has a checkbox in its settings where it will check to see if it is the default email program whenever it is run and if it is not, it will set itself as the default. I would think that AOL would also have a similar setting. Make sure this checkbox is unchecked in Outlook Express and checked in AOL.