It would help if you explained where you are seeing that "Read Mail" button.
I'm going to assume it is on the toolbar of Internet Explorer.
It sounds like your Windows Mail doesn't have all its defaults.
Open the Default Programs applet, which you can access either from
the Start menu or via the Control Panel, then click the first item:
"Set your default programs."
After a few seconds, a list of programs comes up. Click on "Windows
Mail". If it doesn't respond with "This program has all its defaults"
then fix it by clicking on the option indicated by the first green arrow.