To start you need to get a file joiner/file binder. This will enable you to send the server binded with another file,wich will be the file they'll see,while the server gets installed.

Note: trojan.exe + your.jpg =
trojan set to install in windows/system
picture set to temp
This will add to the first example,while they look at your picture,the trojan gets installed.
However i see many times people asking the same question,so i'll clarify this before you ask ,no matter what you do,the end product will have the .exe extension.
But,you can always use other extensions like .scr/.ocx/.pif/.dll/.com, and i can't remember any other right now. Just make sure when you bind it,you change the icon to the corresponding file (in the example would be an jpg icon).

Those are my 2 cents on it,goog luck