Imagine a scenario where internet becomes a "basic need" of a computer. Like a hard drive, internet becomes another source of a great deal of information for your PC. It's not that far fetched dream. Think about the internet TV that's booming today. By next year, we should have TVs for the common user which are powered by streams from internet, instead of local cable service. Hulu has already made it possible, it's only a matter of time before such a service rolls out to different parts of the world.

For the example of an anti-virus scanner. Think about it this way, the application can leverage it's knowledge base directly from its online base instead of your hard drive. Such a scenario will result in a minimal (or no installation) of the software. You need not worry about keeping your definitions up to date because they're pulled directly from the server, remaining up-to-date all the time. Also, deploying new versions of antivirus becomes easier since it's only a loader that's been deployed to an end user machine. Think about how well the vendor could manage it's licenses, piracy can be eliminated almost to the entirety.

I have been researching for sometime in this area. Programming platforms like Adobe Air are working towards making this possible. Already you can develop a complete desktop application - one that can sit in your tray, talk to it's server for all services and still remain cross-platform - using Adobe Air technology. I'm seeing a revolution here, traditional C++ or VB application executable is going to go, atleast to the most extent. Future is of application softwares that consume the least power from your machine.

Check this out: http://www.adobe.com/go/airapp_ebay_learn

That's Adobe's AIR application for eBay. They converted eBay to a desktop software.