Interesting list. It might be informative if we all indicate what version of Windows we're running and whether or not it's 64 bit or 32 bit.
I'm running Windows 7 64-bit and my must have software includes:
Directory OpusClipMateFineprintFirefoxRoboformUSB Safely RemovePDF Converter Professional - use this to view, create, and edit pdfs.
Office 2007 - primarily Outlook, Word, and Excel but have been dabbling with OneNote increasingly of late, though. I think it will probably make the daily-user list next year.
Honourable Mentions:
EditPad ProEndnote - used to use this to generate bibliographies in Word. Now use it to keep my pdf and ebook collection (in the thousands) organzied.
I have TONS of additional software installed, but the first list includes the third party applications that I use every day and the second applications that I use weekly - everything I use weakly

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Edit: added PDF Converter Professional. There *are* other options available but this is the solution I've used for years. Love it's OCR capabilities.
Edit 2: added USB Safely Remove