7zip is the hands down best replacement for winzip. it lets you do way more than the compressed folder wizard in XP, is all available from a handy right click menu, and best of all it's open source. this is a must for shops still using win2000 pro.
VLC media player is the best media player for windows (or any other OS for than matter) it handles a number of pesky codecs and plays DVD's. it too is open source.
clamwin is a widows implementation of the calmAV engine and database for windows. it doesn't do real time file system protection, but you can schedule scans to run in the middle of the night. if you are starting a project quickly or without funds, this is a great way to get passable AV protection without licensing or subscription fees.
autoIT is not open source, but it is free ware and offers simple automation of windows GUI tasks (like simulating keystrokes, clicking buttons, switching windows). it's not as useful as silk or winbatch, but is "good enough" for a great many tasks.
spybot is not open source, and it's status for commercial use is not exactly clear, but i know a number of companies that swear by it for finding, cleaning and immunizing against spyware infestations.
CDburnerXPpro is not open source, but it is free and offers great basic CD burner functionality for burning music and data CD's or DVDs. it's not as great as alchohol 120% for burning movies, but admins and their users shouldn't be doing that at work anyway.