I dunno can't limit it to 5.
I guess the ones I use most often would be ACDsee, Apollo (MP3 player), ICQ, IE, and WinRAR.
But I'd certainly be up the creek without a paddle if I didn't have Windows Explorer or BulletProof FTP (or some FTP...) or a Word Processor or a SpreadSheet proggie. And it would sure suck to not have AudioGrabber to convert my CDs to MP3s...or not be able to watch DVDs without WinDVD...