Some ways to infect someone with a virus or a trojan -


Bind 2 exes (infect a game or any other .exe with your virus/trojan)
A simple thing to do is bind a game with the virus or trojan , lets say you have game.exe and server.exe , there are some programs that will add server.exe into game.exe , so when the program connects those two files it gives you a file.exe that will have both game and server in it , send it to your friends and say its just a game , binder is the program you need to connect these file ....
Send them a downloader
 

A downloader will automaticaly download AND execute any file from the internet on the victims computer as soon as they open the downloader,here it goes:a downloader is 2-4 Kb only! it can be added in a game.exe with the way above,you have to upload your trojan/virus on a server lets say geocities then you set up the downloader to download and run this file,you send the 4Kb file to the victim and as soon his computer runs the file it starts downloading the trojan/virus from geocities,the victim will see nothin, the VIR SCANNERS CANT SEE IT BUT they will detect the trojan/virus that it will download.Downloader is what you are lookin for.....

Infected webpage (.EML Bug)
Another way is to build a webpage that contains a virus and infects the visitors with explorer 5.01-5.5 versions with any virus , i havent test it yet and i am not sure if and how it works but i have seen programs around that promt you to choose a trojan/virus then it decodes it and the it gives you the html that contains the virus the problem is that it takes long time to decode it and its better if your virus is 1-30Kb other ways it ll take days to decode , as i said i havent test it yet and i wont be able to write more or reply to any emails askin for it.

Best way (if you have access on the victims pc)
Get a floppy disc and do the job :-)