Is there any key on the computer that I can assign to enter my e-mail address whenever I want it ?

there isa little programme I used to use which keeps a sort of permanent clipboard, so you can choose which of the items you have on it to paste wherever.
It is Clipomatic. You can get it here (for free)
http://www.download.com/Clipomatic/3000-2384_4-10338261.html

I do hope this was what you wanted.
Pip