Since Outlook 2000 (possibly earlier) you could create an Outlook Template file (.oft) by creating a message and saving as a Outlook template.
There have been many suggestions such as adding to the Outlook toolbar or double clicking the Template file from a folder with your saved templates.
Lets take this one step further with Windows 7 jumplists and integrate into the toolbar.

Figure: Jumplists are shown by right clicking on icon on toolbar
First lets create a template by creating a New email

Figure: A Meeting Agenda is a good template to have
Once we have created this Click File > Save As and store in either the default template location of under your documents

Figure: Outlook 2010 Save Dialog but this will work in Outlook 2003/2007 as well
Change the File type to .oft (Outlook Template file)

Figure: Save As dialog. Change the save as type
Now open the folder in Windows Explorer where you saved the template and drag onto the Outlook icon pinned onto your taskbar (Pin Outlook if you haven’t already first). If you drag onto a blank part of the taskbar you will get same result.

Now when you right click Outlook you can select the Template pinned to the top of the jumplist and with 1 click open Outlook and launch new email with your template.

Happy trails!