Where in the world are you located? Some versions of Windows 7 do not come with Internet Explorer so there is no default email client. IE8 is the default browser in USA versions of Windows 7. Go to the link below to download IE8 from a network enabled computer. Transfer the files to your computer and install IE8. After IE8 is installed you can use any email client you want.
Any Email client you install will become the default Email client if you make it so. You don't necessarily need to install Internet Explorer Stuart
You can use any mail client and it will become your default email client. I use Mozilla Thunderbird for my email, it is good, it is free, it works much like the old Outlook I used for many years before. In the old days, Outlook Express came with all Windows machines and Outlook came with Office so you upgraded if you used Office or if you were comfortable with it at work. Then Outlook only came with the more expensive versions of Office and later, Outlook Express was dropped. I tried Thunderbird on a recommendation and have never looked back.
Some like Windows Live Mail as their client. I have not tried it.You have to be a little bit crazy to keep you from going insane.
