What programs can't you live without? They can be really big, important things, or just silly things that you've grown accustomed to, and can't see yourself without it.
The big one for me is Notepad++. I love the speed and compactness to it. As a programmer, I love the different features that come with it, such as syntax highlighting and (limited) intellisense/auto-completion. I also love the Compare function to it. It's saved my ass as a programmer many times.
It's also just a really good replacement for Notepad. Great for reading and writing text files, scripts, bat files, etc.
Another program I need to have is Winamp. I know other things like WMP do a lot more, but Winamp is the best mix of form and function for me. I love the speed of the search, as well.
Finally, Media Player Classic. Great free player for watching movies. Kicks the ass of every other movie player I've seen.