How to create a patch file on windows?

Try WinMerge. You'll find a patch generator in the "Tools" menu.

Thanks, PhiLho & Lukáš! The diff program from UnxUtils works great under Windows 7 to generate the patch file:

diff -u oldfile newfile >patchfile

Likewise, the patch program from UnxUtils works great to apply the patch:

patch -u oldfile patchfile