Not able to install any package on Ubuntu 9.04

Starting a new tech stream on my blog to share my knowledge. And of course, it would help me when I come across the same issue in future. Here goes my first post.

I tried to install Ubuntu 9.04 using Wubi today. Wubi really made my work a lot simpler than installing Ubuntu on a separate partition. For those who don't know about Wubi, its a Ubuntu installer for Windows. It just creates a file in Windows for the size you specify (min of 5GB) and installs Ubuntu there. You can uninstall it as a normal program from Add/Remove programs feature.
The complete installation process (Ubuntu download + installation setup) was done in 30 minutes. But to my dismay, I was not able to install any new package or software either through apt-get or synaptic package manager. I wasn't even able to install vim due to a stupid error which I did not understand.

$sudo apt-get install vim

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package vim is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package vim has no installation candidate

The same error is thrown for all packages. After some googling, I got rid of this error by doing an update for the package manager.

$sudo apt-get update

-this command updated the package manager and thereafter I was able to install any new package thru synaptic and apt-get as well.