You can install WinMerge by downloading the latest version from their website at the WinMerge Downloads Page
After installing, run it.
To compare files press CTRL-O (File | Open) and choose the desired individual files to compare.
Or, drag-and-drop files from an explorer window onto the WinMerge window. The 2nd file you drag onto WinMerge needs to be dragged onto the 2nd filename box.
Click OK, and the files' contents will be compared, with differences highlighted.
There are arrow buttons in the navigation bar to allow you to move down and up thru known differences, and left/right to copy text from one to the other.
ESC exits. If you've made any changes, you'll be prompted to save them, for both files (if relevant).
To compare entire folders, do the same as for files above, but only enter folder names (or drag-and-drop). BE SURE TO CHECK THE "Recursive" CHECKBOX so that it knows to compare files in subdirectories too.
You'll see a list of files after comparison. Under the "View" menu, deselect "Identical Files". You can optionally enable/disable left/right comparison results too.
To look at diff's in displayed files, double-click or press Enter on a highlighted entry. Edit as described above for individual file diff's.
Choose Tools | Filters | Linefilters tab and check the "Ignore lines..." box, and enter this:
(Prior to WinMerge v2.2, choose: Edit | Options | Filters and check the RegEx box)
This will tell WinMerge to ignore commented text as well as common header components in each file.
This allows comparing between ZC release versions without having to worry about files that only have changes in the headers or in commented text.