Unlike Vista & WinXP, Win7 doesn't include a default Mail Client. [What were they thinking?]
You will have to install one (e.g., MS Outlook; Windows Live Mail; Thunderbird) and then set it as your default for Mail in the CUSTOM (<=important!) section of Set Program Access and Computer Defaults and then restart your computer before any Send To or MailTo functions will become available.
You will find specific support for Outlook in this forum: http://answers.microsoft.com/en-us/office/forum/outlook
You will find specific support for Windows Live Mail in these forums, at least until 29 March 2012, 00:00 UTC: