How to make users download your software

In the past couple of months other start ups and my self are wondering, "with all the web 2.0 out there does users still download/install desktop application".
What are the desktop app who made it in the last 4 years?
If users do download, then who are does people? and what makes them download?
is it a friend recommendation?
A blog post?
A lot of time i hear start ups says: our software is "Viral" (and most time it is don't)
I wrote down a few pointers for the desktop application developer:

1. Does it worth something if im not online?
   I think this is the first thing you have to ask your self before you design a desktop application.
2. Does it have an extra value has a desktop application?
   if your application can do anything it does as a web app so make it a web app.
3. Can your application deliver a cool GUI that can't be delivered in an online app?
4. Will your users know and like to install desktop apps.

Thats my points before you decide to develop a desktop app and not an online one.