Why do computer viruses have to be all gloom and doom? There should be fun viruses!
Like how about digital "drugs"? You download a little app. Like taking a pill of your drug of choice.
It infects your operating system for 4 - 5 hours. During that time, everything looks psychedelic: icons "trail" when you drag them. Rainbow colors and mirages appear. When you type, it comes out different than you expected, but much more hilarious.
Would be great as a prank, too.
Special thanks to Kassy for helping brainstorm this one.