Delete(Or put it in another folder.) one of his icons (If he has an anti-virus icon, replace it with that. Otherwise, use the most used icon.)
Go to notepad, type this:
do
onclick = msgbox("Your system32 folder has detected 154 viruses. This program was unable to execute. Please contact your nearest PC technician or format your Hard Drive.",16,"Fatal System Error")
a = msgbox("Fatal Error",16,"Error")
loop
You can change those to whatever you want. It will play both of those and never leave.
Now, place that in any folder of your choice. How about...Pictures? Music? Documents? research
Create a shortcut for that.
Place it on the desktop and right click it, go to properties. Click the tabs 'till you find Change Icon.
Click it (Durr)
Navigate to the location of the icon you replaced with this. (If it was CSS icon, go to your Steam folder and find the .exe for it.
Double click the .exe and click the icon that appears.
Click apply, then OK
Now it looks like CSS (Or any other icon you replaced it with.
Just...walk away.
Watch him click it.
Watch him be like "WTF IS HAPPENING!!? FFFFFFUUUUUU!!!"
???
Profit!
:D