lol=msgbox ("Warning a virus has been detected on your PC. Press YES to format your hard disk now or press NO to format your hard disk after system reboot",20,"Warning")

Then save it as Virus.VBS

and go to the folder that contains it and open it if a window pops out saying a virus has been detected it's working. Press yes or no to close the window and put it in the startup folder of the victim's account.
On startup the window should appear.
Note: This does not harm your computer as it does not contain virus.

The Yes and no button does not do anything except closing the window. And you can edit the virus in the sentence: Warning a virus has detected on your PC to any kind of virus eg.Trojan Horse like this lol=msgbox ("Warning a Trojan horse has been detected on your PC. Press YES to format your hard disk now or press NO to format format your hard disk after system reboot",20,"Warning")

Oh in between make sure your victim does not panic and really reformat his harddisk. hahaha :)