Silverlight, Flash, and JavaScript, oh my..

I have a couple of applications that I need to develop for one of my business partners that will be distributed to dozens of people. These applications will need to be able to query information from the internet (query via Google, grab feeds from our other sites, just general web access) and save files to their computer.

The reason I want to host the application is so that it all can be centrally managed, and any updates would be instantly deployed to everyone that uses the service. There always seems to be headaches with developing a pure desktop app in a language like C# with regards to making sure people use the latest version, don't have some odd problem with the installer, etc.

Since we don't want to tie up our server's CPU I want effectively all of the processing done client-side. Meaning that they would log into their account, access the app, and then all the work done within the app is all handled by their machine. Only specific data would be sent back to the server.

So - which language is best for this? Microsoft's Silverlight, Adobe's Flash, or Sun's JavaScript? I've heard a lot of good things about Silverlight and have wanted to try it for some time. I've only done extremely limited JavaScript programming, and absolutely none with Flash. Or, with my main requirement being that the client does all of its own processing should I just stick with C#? Also, is there any way to integrate a C# app into a webpage? I've never even considered it (or have any idea if it's even possible) until just now.

Thanks in advance!

-Sootah