On XP, if you want to reassign the .zip format to be opened by XP's own zip program (instead of Winzip):
- Go to your control panel (thru start menu)
- double-click on Folder Options (new window opens)
- select the File Types tab
- scroll down to ZIP and select it
- below that in 'Details for ZIP extension' click on the Change button
- in the 'Open With' window, under Recommended Programs, select "Compressed (zipped) Folders"
AND
- make sure you checkmark the "Always use the selected program to open this kind of file" which is just below.
- click OK
Now, whenever you have a zip file...just double click the zip, it'll show you what's inside, you can either extract to the same folder, or tell windows where you want to extract...in the case of the iconpackager, you can navigate to the themes folder. XP
should create a new folder for it there.
Sorry...that's a little long winded, and there are easier ways to do it...but that should get your zip files associated with the XP program...you can always choose to open a zip with Winzip any time.

hope that helps somehow!