Windows version?
OE version?
What makes you think it was removed?

Control Panel | Add or Remove Programs
On the left side there should be something about Windows Components.  Open
that and make sure OE is checked.

It's not likely that it got deleted.

Some other mail program might have made itself the default mail program.
To set Outlook Express back as the default mail program:
Start, Run and enter:

 "C:\Program Files\Outlook Express\msimn.exe"   /reg

To recreate missing shortcuts to start OE, create a shortcut with the
target:

 "C:\Program Files\Outlook Express\msimn.exe"