Must-Have Windows Software (or Windows Programs that I use)

Nice list, I will just add some of my tools you didn't mention, as in general, I can totally support your list, I use many of your tools, too.

FreePDF
Great PDF printer GUI based on Ghostscript

PSPad
Yet another notepad++, I like it more than notepad++, maybe just from knowing it better and having an integrated Diff, HexViewer and FTP client

ClipX
You mentioned it, I really rely on having a clipbard history sometimes, but it has an annoying problem with OpenOffice:
Sometimes when you copy in OpenOffice, it (OOo) freezes totally. Sometimes it helps killing ClipX to unfreeze, sometimes not.
I just today downloaded the beta version (1.0.3.9) to try, maybe that will help.

IrfanView
This is my favorite image viewer, also good for simply changes of resolution or converting...

Gimp
For the bigger imaging tasks

VLC
do I need to describe this? ;)

Audacity
What gimp is for images and VirtualDub is for video, Audacity is for audio.

MyExposé
Nice tool to show your open windows like the mac does, nicely zoomed and sorted on the screen with one keypress.

BrowserChooser
Set up as default browser, you get n buttons when opening an URL to choose wich browser you want to use this time. The only downer it its loading time as it seems to be an c# WPF application...

KDiff3
This is my diffing tool, also used in juction with subversion and TFS merging procedures.
Nothing I have seen up to now makes the 3-way-diff that clear and easy to perform.

XMing
You use ssh, for instance with putty? You want to see an x window, but don't whant full cygwin on your machine? This will help you out: this simple exe is all you need.

VirtualBox
Well, thats my favorite virtual machine. I wrote and occasionally update my tool CloneVDIGUI as gui wrapper for vboxmanage clonehd.