Here’s a little trick you can play on a buddy if you have access to his computer. We are going to create a popup dialogue box warning of the presence of a virus. First, open up notepad and type the following line,
X = msgbox("Your Computer has been infected by a virus. Immediate action required!", ,"WINDOWS SECURITY ALERT!")
Save the file with whatever name you want but make sure it has the extension .VBS
Next, right-click the Start Menu button and navigate to Explore All Users > Programs > Startup and place your newly saved file there. This will activate it the next time the machine is booted. If you want to test it simply open it. This is what it should look like,

If you really want to be irritating copy that line as many times as you can. For instance, if you save the file like this …
X = msgbox("Your Computer has been infected by a virus. Immediate action required!", ,"WINDOWS SECURITY ALERT!")
X = msgbox("Your Computer has been infected by a virus. Immediate action required!", ,"WINDOWS SECURITY ALERT!")
X = msgbox("Your Computer has been infected by a virus. Immediate action required!", ,"WINDOWS SECURITY ALERT!")
… it will create the popup a total of 3 times, one for each time the popup is closed. Obviously the more lines you add the more the popup will appear.
You can use this technique to create any type of message you want. Use your imagination.
If you want some ready made pranks to run RJL Software has a fairly good selection to choose from. A word of warning if you decide to use any of them – most modern antivirus apps will flag them because they are considered nuisance software. They are not malicious.